Discordant zircons from the Little Belt (Montana), Beartooth (Montana) and Santa Catalina (Arizona) Mountains

Deochimica et Coamochimica Acta, 1964, Vol. 28, pp. 87 to 124. Pergamon PressLtd. Printedin NorthernIreland

Discordant zircons from the Little Belt (Montana), Beartooth (Montana) and Santa Catalina (Arizona) Mountains* E. J. CATANZARO~

and J. L. KULP$

(Received 6 March 1963; in reviae.dform 1 July 1963) Ah&&-U-Pb and K-Ar isotopic age analyses were performed on zircon, monazite, biotite and hornblende samples from the Little Belt and Beartooth Mountains of Montana, and the Catalina Mountains of Arizona. In the Little Belt Mountains, three events were dated: formation or resetting, of zircons 22500 m.y. ago; metamorphism, 1920 m.y. ago; and intrusion of Laramide porphyries, 120 m.y. ago. In the Beartooth Mountains, the results indicate that the detrital zircons found in the Precambrian me&sedimentary rocks are more than 3100 m.y. old. In the Catalina Mountains, the results suggest that the Catalina gneiss was initially formed -1650 m.y. ago and was remetamorphosed 30-40 m.y. ago. The zircons from all three areas give highly discordant isotopic ages. The discordances can be accounted for by either episodic lead loss or isotopic fractionation followed by ground water removal, but are incompatible with the hypothesis that continuous diffusion is the predominant mechanism. INTRODUCTION

common occurrence of discordant results in isotopic geochronometry presents an intriguing and complicated problem. It has become obvious that many mineral samples used in age determinations have not been closed systems throughout their histories. The interpretation of isotopic ages ultimately requires knowledge of the processes which can cause alteration of the isotopic ratios. The primary purpose of the present study was to examine the causes and nature of the alteration of the U-Pb system in zircon, one of the most useful minerals for geochronology. Theoretical considerations (WETHERILL, 1956 a, b) had suggested the importance of analyses of multiple samples of zircon from specific geologic settings. Experimentally the utilization of zircons from ancient rocks would provide maximum sensitivity. Three areas in the Rocky Mountain region were chosen because they appeared to have had geologic histories which would yield unique information for the problem. The Santa Catalina Mountains represent an old basement that was subjected to regional metamorphism during the Tertiary period. The Beartooth Mountains contain some of the oldest rocks so far identified in North America, and previous studies (GAST et al., 1958) had shown that the last metamorphism in this area had occurred in very ancient time-2700 m.y. ago. Also, since the zircons in these metasedimentary and granitized rocks are predominantly detrital (ECKELMANN and POLDERVAART, 1957), it was considered possible that their isotopic ages might reflect the age of an older event.

THE

* Lamont Geological Observatory Contribution No. 662. t Geochemical Laboratory, Lamont Geological Observatory; present, address: Bureau of Standards, Washington, D.C. $ Department of Geology, Columbia University, Palisades, N.Y. 87

National

E. J. CATANZABO

68

The Little Belt Mountains contain a are cut by Cretaceous intmsives. This study the effect of young intrusives on study progressed, maximum attention

and 5. L. KULP

core of ancient gneisses and schists which appeared to be an ideal area in which to the U-Pb systems of old zircons. As the was given to this area because of the

interesting geological and geochemical problem which developed. A natural by-product of this investigation was the determination ages of geologic events in all three areas. GE~LOQIC~L LMe

Belt

of the absolute

SETTINGS

Moulztains, Montana

The Little Belt Mountains are located in Cascade County in central Montana and are the easternmost expression of the Rocky Mountains in this area. Older Precambrian (pre-Belt) rocksoutcropin two large areas in these mountains. Figure 1

a

Syenite 8 porphyry sy Barker porphyry bp White porphyry wp Neihart porphyry np Felsic porphyry dikes & sheets

&j

Barker formation

m m a a cz3 0

Be%%%? Ab Pinto metadiorite pd Phyllltes 6 schists Augen gnelss Grey goeiss Mlgmatlte Undlffkrentiated pre-Bel$ag;rocks

I/ I/

Area mopped in preser;t study

?B a B

Fb

Scala 012345 MILES

Fig. 1. Geologic map of the N&art

8R38, Little Belt MOLId8h3.

is a small scale geologic sketch map of the northeaetern area of exposed Preoambrian rocks. In the present study a narrow zone, immediately adjacent to U.S. Highway 89 (which follows Belt Creek), was mapped in a preliminary attempt to differentiate the older Precambrian rocks (Fig.2). This particular traverse was appropriate because the rocks are almost continuously exposed over a distance of approximately 10 miles perpendicular to strike. The strike of the layering in the older Precambrian rocks along Belt Creek is seen to be locally variable (Fig. 2), but on a largescale it is essentially uniform in an E-W direction. Dips are also locally variable but yield a definite pattern on a large sea&. North of the Pinto metadiorite exposure, the dips of the layering in the migmatite

Discordant zircons from the Little Belt, Beartooth and Santa Catalina Mountains

89

CRFTACFOUS Felsic wrphyry

K I-

K IO

Fig. 2.

LAMBRIA&

m

Pinto melodiartte

m

Phylliies

m

Augen gneiss

0

Grey qneiss

8 schists

Geologic outcrop map of a portion of the Neihart Precambrian Little Belt Mountains.

exposure,

are vertical or to the north. They average 75’N (29 per cent vertical) in the northernmost 3.8 miles of exposure, and 45’N (0 per cent vertical) in the 1.6 miles of exposure immediately north of the Pinto metadiorite. Just south of the Pinto metadiorite, in the grey gneiss exposure north of Neihart, 50 per cent of the dips ~4 miles of exposure, the dips are predominare vertical. In the southernmost The major structure appears antly to the south, averaging 55’S (3 per cent vertical). to be an anticlinorium.

90

E. J. CATANZARO and J. L. BULP

No evidence of large scale faulting was observed along the major traverse but a highly sheared chloritized fault zone is exposed in an outcrop along the Hoover Creek Road in the northern part of the area. The major fractures in the outcr,op No attempt was made to trace the appear to strike approximately north-south. The area immediately to the south is swampy shear zone in a northerly direction. and good exposures are absent. No evidence of a shear zone was found in the area in which the projected strike of the zone crossed Belt Creek. I’etrograph y. The basement rocks can be divided into the older gneisses and schists and the younger, but metamorphosed, diorite stock (Pinto) and basaltic dikes (WEED, 1900 and SCHAFER, 1935). In the area that was mapped during this investigation the units were taken 8s migmatite, grey gneiss, augen gneiss, schistose rock, metadiorite and metabasalt. The gneisses, schists and migmatite were somewhat interlayered. The map pattern indicates the dominant rock type of the area. Migmaiite. The migmatite is a highly contorted heterogeneous gneiss with segregated mafic and felsic layers (generally a--2-in. thick). The major minerals noted are sodic plegiocla~, microcline, green-brown biotite, hornblende and quartz. Locally (Fig. 2) garnet is present, along with deep red-brown biotite, muscovite, sodic plagioclase and quartz. One migmatite sample showed a striking varicttion, consisting of augitc, hornblende, bi0tit.e and endesine (Fig. 3a). The accessory minerals are epidote, apstite, sphene, zircon, opaques end occasionally, Principal alteration products are sericite and chlorite. Quartz end feldspar grains monazite. sometimes show undulatory extinction and plagioclase twin lamellae are occasionally bent. The grey gneiss is simply banded showing only lOC81drag folds and involutions. The major minerals present are green-brown biotite, sodic plagioclase, potash feldspar and quartz. Some grains of potash feldspar show only simple grid-twinned microcline; others show Carlsbad twinning and only minor, spotty, grid twinning, suggesting only partial conversion of orthoclase to microcline. Accessory minerals noted include apatite, epidote, spheric, zircon and opaqia~ minerals. Sericite and chlorite are present as alteration products. The rocks show evidence of cataclastic deformation in plagioclase grainswith steplike displacement of twin lamellae (Fig. 3b). Quartz and feldspar grains show undulatory extinction. The augen gneiss (SCHAFER’s “red gneiss”) can be differentiated into three phases which appear to be genetically related and differ only in the development end size of augen: (1) A rock with regularly alternating pink (quartzo-feldspathic) and dark green (biotitc! and chlorite) layers with small and minor augen. (2) A rock containing similar alternating layers but moderate sized (t-4 in.) pink (feldspar) augen are present in the dark layers. (3) A normal augen gneiss with dark groundmass (biotite and chlorite) and large (i-1 in.) pink feldspar augen. Layering PI no longer conspicuous. Tho major minerals noted in the auger1 gneiss are orthorlsse, microclirs~, sodic plagioclase, quartz, biotite and chlorite. Minor minerals include epidote, sphene, apatite, zircon and opaques. Sericite is an alteration product and small muscovite (aftnr sericiti) laths are occasionally noted. Carbonate veinlets are common. The augen (orthoclrtqo, microcline, minor albite) are simple, generally anhedral, grains slightly otongate parctllel to the foliation. In hand specimens, some of the larger augen show the form and typical Carlsbad twinning of orthoclase. In thin-section. patchy grid-twinning is noted urn1 it appears as if, in general, the microcline is inverted orthoclase. Chlorite and green biotitc occur in streaks of thin laths which curve around, and are squeezed between, the augen. Quart,z occurs in streaks of small anhedral grains interstitial to the augen. Microscopically the texture of the eugen gneiss rocks reflecti cataclastic deformation. The borders of the augen grains em serrated and surrounded by smaller grams which appear to have broken off from the main large grain. The eugen generally show undulatory extinction and some plagioclase grains have displaced twin lamellae. A fourth phase included in the augen gneiss on the geologic map is e red microcline-quartzaibite rock without prominent gneissic structure. Outcrops of this granitic rock have a general dikelike appearance and although the rock is abundant only in the eugen gneiss region near Neihart, rare outcrops of he same rock type also occur in the migmatite area to the north. In the area mapped as augen gneiss, this granitic rock contains microcline, orthoclsse, quartz,

Fig. 3. (A) Augite crystats-migmatite l(Little Belt Mountains), (B) displaced plagioclese twin lamellae-grey gneiss (Little Belt Mountains), (C) feldspar augen-granitic phase of augen gneiss (Little Belt Mountains), and (D) feldspar augen-phyllite (Little Belt Mountains).

so

Fig. 4. (A) Feldspar augen~hyllite (Little Belt Mount&~), (B) zircons showing simple and multiple bipymmid terminations-migmatite (Little Belt Mountctins), (C) stubby, multifaceted ziroon-migmatka (Little Belt Mountains) and (D) zircon with be& pinacoid-augen gneiss (Little Belt Mountains).

Fig. 5. (A) Zircon with numerous expansion cracks-migmat,ite (Little Belt Mountains), (B) malformed zircons-migmatite (Little Belt Mountains), (C) malformed zircons--grey gneiss (Little Belt Mountains) and (D) double zircon grain with outgrowth-granite gneiss (Beartooth Mountains).

Fig. 8. (A) Zircon with outgrowth+ta gneiss (Beartooth Mountains), (B) zircon with outgrowth-granite gneies (Beertooth Mounteins), (C) zircons from Catalina gneiss and (D) zircons from C&tdina gneiss.

Discordant

zircons from the Little

Belt,

Beartooth

and Santa Catalina

Mountains

91

biotite and sodic plagioclase. Undulatory extinction and displaced twin lamellae are common. In thin-section some feldspar and quartz grains are elongate and have an appearance similar to augen {Fig. 30). Thin-sections of this rock type from outcrops in the migmatite area show almost only microcline and quartz (minor albite and biotite) and a more granitic texture. Undulatory extinction of quartz and feldspar grams is common. The schistose rocks outcrop in a restricted area (Fig. 2) and generally contain quartz, sodic plagioclase, orthoclase (microcline), biotite, and chlorite. Occasionally muscovite is present, and in one poor outcrop, garnet was found. Minor minerals present include epidote, apatite, zircon, sphene and opaques. Sericite occurs as an alteration product. The rocks have a varied appearance in hand specimen (from outcrop to outcrop). They range in appearance from chlorite and biotite schists to quartzose phyllites. However, thinsections indicate that these rocks should be classified as phyllonites. Although generally finegrained, they contain streaks and pods of larger serrated grains (feldspar) which appear to be remnants of cataclastically deformed larger grains (Fig. 3d and Fig. 4a). The streaks of mica and chlorite curve around the podlike “remnants” just as the biotita and chlorite curve around the augen in the augen gneiss. Undulatory extinction and bent twin lamellae are common. The Pinto metadiorite was intruded into the older rocks prior to the last metamo~hism. A good description of this rock can be found in PIRSSON (1900). The principal minerals are andesine, biotite and hornblende, with minor amounts of orthoclase, quartz, apatite, sphene, epidote and opaques. Carbonate veinlets are occasionally present. A remarkable aspect of this rock is its pseudo-porphyritic structure (PIRSSON, 1900). It contains augen that does not consist of simple feldspar crystals but of a number of such crystals differently oriented. The biotite and hornblende fill the interstices between the augen. Acnording to PIRSSON (1900) the Pinto diorite has a porph~itie fine-grained border phase which contains true feldspar phenoerysts and is more highly enriched in dark minerals with respect to the main body. A small region within the mapped area of Pinto diorite (Fig. 2) appears to be an “island” of grry gneiss. The rock here is variable from outcrop to outcrop and appears to bc a mixture of gneiss, meta-amphibolite and Pinto diorite border phase. Zircon. The shapes of the zircons from the Little Belt Mountain area (Figs. 3 and 4) even within a single suite, are variable. Some grains appear rounded and corroded; others have plane faces and sharply pointed terminations. Some terminations are simple bipyramids, others are multifaceted bipyramids and others are basal pinacoids (Fig. 4b, c, d). The grains are all moderately or extensively metamict and expansion cracks are common (Fig. 5a). There are no obvious outgrowths or complete overgrowths, but some crystals have bent prisms and bulges (Fig. 5b, c) which may represent deformation and minor reconstitution. Occasional grains have rims which differ in birefringence from the cores. In a few suites from the migmatite rocks there appear to be ~adations between two different types (Fig. 4b): dark, stubby, multifaceted zircons; and clear prismatic zircons usually with simple, sharp bipyramid terminations or (rarely) basal pinacoids. The four zircon populations from the northern 4000 ft of the area (15, 27, 26, 14) (Fig. 2) show similar morphology (statistically), and therefore may represent a mixture of detrital grains from a single rock complex (with suitable mixing) or may have crystallized in similar igneous environments in their present location. Sample X3 occurs about one mile sout-h of this group and shows growth characteristics sig~i~cantly different from the other four. Even though 13 has a similar host rock, now, it apparently was derived from a different source than the other four. There is no evidence of overgrowths among the zircons from the entire Little Belt area so that the differences were presumably not developed during a metamorphism. The two samples from the grey gneiss (31, 41) have similar growth characteristics, even though widely separated, and may have been derived from the same igneous body or rock complex. The growth characteristics of the zircons from the augen gneiss sample (34) are different from those of the grey gneiss suites and it would appear that they originally formed in a different environment. One phase of the augen gneiss is characterized by uniformly shaped and sharply terminat,ed zircons such as is shown in Fig. 3(d).

92

E.

.J.

('ATANZARO

and

J. L.

KULP

of

nature of the basement rocks IR Pte-mehmorphic chaurmcter rocka. The pm-metamorphic Both WEED (1900) and SCHAFER (1936) concluded that all of the rocks are metauncertain. igneous because of a lack of evidence of any primary sedimentary structures. However, the metamorphism of these rocks has obliterated most of the obvious evidence for either type of origin. One sample of migmatite contains augite, hornblende, biotite and andeeine. This assemblage is consistent with a metamorphosed mafic igneous rock but without further chemical study thu evidence is not diagnostic. Other mineral assemblages found in the migmatite rocks are consistent with the amphibolite metamorphic facie8 (WILLIAMS el al., 1968), and are not diagnostic: with respect to origin. The finely segregated layering, and the highly variable thickness, spacing and composition of the bands of migmatite suggests a sedimentary origin. It is dimcult to see how initially homogeneous (9) intrusive igneous rocks could develop such a finely layered metamorphic character. Although the formation of layering in igneous rocks by metamorphic segregation (aside rocks) is not well documented in the literature, a recent study of the striped from “flaser” amphibolites of Connemara (Ireland) is pertinent.. The layering in these rocks, except for a lack of strong contortion, is similar in scale to that of the migmatites. EVANS end LEAKE (1960), on the basis of extensive chemical studies, concluded that these layered meta-amphibolites were originally igneous rocks (intrusives, lavas or tuffs or sodic basalt composition) and the layering was due to intense metamorphic segregation along schistosity planes. As previously mentioned the zircons in all of the rocks are variable in morphological charaoter and degree of crystallinity; they are noither definitely “igneous” nor definitely “detrital”. However, the results of the statistical studies show that the grey gneiss zircon suites are similar to each other but different from the augen gneiss zircon suite;. The fact that the augen gnoiss suite is located between the two gray gneiss suites (Fig. 2) must be explained. Since it is fairly ~011 established that metamorphic events do not alter the shapes of zircons, excluding ultra. metamorphism and overgrowth formation which are not in evidence here, the situation can be explained by three alternatives: (1) igneous zircons from different sources were transported to thoir present location at different times. This h_ypothesis must include the corollary that there ix a stratigraphic difference between the two rock types; (2) the augen gneiss is igneous in origin and was intruded into the rocks which now compose the grey gneiss; (3) both rocks were originally igneous and the zircons they contain are different because of different crystallization environments. Tho uniform layering in some phases of the augen gneiss and the preponderance of ono type of sharply euhedral zircon in the granitic phase of the augen gneiss strongly suggest an igneous origin for this rock. Some small outcrops in tho northern part of the mapped area contain both migmatite and augen gneiss types. If both of these rocks are considered to be meta-sedimentary, the difference between the two would presumably be due to minor variations in the original sediments. It is concluded that the original charactor of the migmatite and grey gneiss rocks prior to the first metamorphic event cannot be determined with certainty, but the augen gneiss phases appear to have been granitic igneous intrusions. Since the schistose rocks were probably formed by sever0 cetaclastic deformation of the augen gneiss (or its precursor) their original character was probably similar to that of the augen gneiss.

Geologic history ofthe area. The original material either detrital or igneous filled a geosyncline. This was thoroughly metamorphosed and probably intruded by granitic rocks which might represent the precursor of the augen gneiss. The metamorphism was followed by intrusion of the Pinto diorite and basaltic dikes which sharply cut the plastically deformed banding of the older metamorphic rooks. A second period of metamorphism converted the granitic bodies to augen gneiss, the mafic intrusives to meta-diorite and meta-amphibolite and produced felsic dikes and veins which cut across the Pinto meta-diorite and meta-amphibolites.

Discordant

zircons from the Little Belt, Beartooth

and Santa Catalina Mountains

93

Subsequently, all of the pre-Beltian rocks were subjected to cataclastic deformation of various degrees producing serrated grains, displaced twin lamellae and undulatory extinction. It is possible that the variation in degree of cataclastic deformation is a simple function of proximity to a shear zone. This may account for the severe deformation of the schistose rocks, but the presence of grains with undulatory extinction even in northern exposures of migmatite suggests that the deformation was of broader extent. After uplift and erosion, the area was covered by water and Belt sediments were deposited. After another cycle of uplift and erosion, Cambrian and Devonian sediments were deposited. The area was extensively intruded by porphyry dikes and plugs in Cretaceous time. Finally, uplift and erosion produced the present setting. Beartooth Mountains,

Montana

The Beartooth Mountains are located on the Wyoming-Montana border just north of Yellowstone National Park. They are one of a number of ranges which

Fig. 6.

Index map of the Beartooth

Mountains.

surround the topographic and structural Bighorn Basin of Wyoming and Montana (Fig. 6). Structurally, the Beartooth Mountains form an archlike uplift bounded by the Beartooth thrust to the northeast and the Gardner fault to the southwest. Differences in thickness of sediments in the area yield evidence of repeated displacements between basin and mountain blocks since at least earliest Paleozoic times (HARRIS, 1959). The present topography is a result of the Laramide Revolution, during which the Beartooth block was tilted to the southwest and thrust northwestward (BUCHER et al., 1934). A detailed study of the structure of the Beartooth Mountains is given by FOOSE et al. (1961). The seven Beartooth Mountain zircon samples analysed in this study were collected from three different areas (Fig. 6): one from the Gardner Lake area (GL),

94

E. J. CATANZARO and J. L. KULP

two from the Stillwater River area (SG and 7) and four from the Quad Creek area (1, 4, 5, 6). The Quad Creek area was chosen for multiple sample study because of its accessibility and the availability of a detailed geologic map (ECHELD~ANNand POLDERVAART, 1957). Figure 7 is a portion of the ECKELMANN-POLDERVAART geologic map, showing the location of the analysed zircon samples. These samples were chosen in an attempt to determine if rock type and/or proximity to Laramide intrusives had significant effects on the discordancy of the zircon ages.

Fig. 7. Geologic map of a portion of the Quad Creek area, Beartooth Mountain8 (after E~ELMANN and POLDERVAART, 1957). The dominant structure in this area is a syncline trending 5*N-10% and plunging lo”-3O”SW. The structure is continuous across a gradational boundary zone between migmatites and m&a-sedimentary rocks to the northeast and granite Wherever observed, foliation in the granite gneiss and gneiss to the southwest. migmatite parallels bedding in the meta-sedimentary rocks. The major rock units in the area arc Precambrian granite gneiss, migmatite, meta-sedimentary rocks (primarily para-amphibolites and quartzites), and metaigneous mafic dikes and stocks. Felsic Laramide porphyries are presented and have been described in ROUSE et a2. (1937). ECKELMANN and POLDERVAART (1967) present evidence indicating that the Precambrian rocks experienced a period of granitization just after they were The subjected to regional metamorphism of the highest amphibolite facies. sequence of events according to these worker8 is: ( 1) deposition of ancient sediments, (2) folding, (3) metamorphism and granitization, (4) uplift, (5) peneplanation

Discordant

zircons from the Litt.le Belt, Beartooth

and Santa Catalina Mountains

95

(6) deposition of Paleozoic and younger sediments and (7) uplift, thrusting and emplacement of felsic porphyry dikes. The zircons from the Precambrian rocks of the Quad Creek area are predominantly rounded, suggesting a detrital origin, but show abundant overgrowth phenomena (Fig. 5d, Fig. 8a, b). Detailed studies (POLDERVAART and ECKELMANK, 1955; ECKELMANN and POLDERVAART, 1957) have shown that the abundance and completeness of overgrowths were greatest in the granite gneiss and least in the quartzite. They consider these results as substantiating evidence for the theory that the granite gneiss was formed in situ from sediments. Santa

Catalina Mountains,

Arizona

The Santa Catalina Mountains are located immediately north of Tucson: Arizona. They are the northern part of a group of ranges that includes the Tanque Verde and Rincon Mountains to the southeast. The core of the Santa Catalina Mountains is a granitic-gneissic complex bounded on the north by sedimentary rocks, on the east by younger Precambrian (Apache group), P a1eozoic and Cretaceous (2) sedimentary rocks, and on the west The geologic ages and stratigraphic and southwest by meta-sedimentary rocks. relations of the gneissic and meta-sedimentary rocks are discussed by DUBOIS (1959 a, b). The oldest meta-sedimentary rock is the pre-Apache Pinal schist. The gneisses are also pre-Apache but their relation to the Pinal schist is not known with certainty. The Catalina gneiss outcrops in the south central portion of the Santa Catalina Mountains. DUBOIS (195913) distinguishes three broad types: banded augen gneiss, augen gneiss and granitic gneiss. The mineralogy of all three types is similar (primarily quartz, plagioclase, orthoclase microcline, muscovite and biotite) and boundaries are gradational. In most places the Catalina gneiss has a single pronounced planar and linear structure, but locally there is also a second, less prominent planar and linear structure. The structures are indicated by oriented mica and feldspar crystals as well as by oriented mylonitized material (DuBo~s, 195913). DUBOIS interprets the structure as evidence of two periods of metamorphism, with the latter one being more pronounced and responsible for the present gneissic structure. He dates the earlier metamorphism as Precambrian and the later as post-Cretaceous. The zircons dated in this study were concentrated from a 300 lb. sample of gneiss supplied by Professor PAUL E. DAMON of the University of Arizona. The sample was collected at the Hitchcock Memorial on the Mount Lemmon Highway in Pima Country. The zircon suite consists of essentially two types (Fig. 8 c, d) clear euhedral prisms with sharp bipyramid terminations and high birefringence: darker, less clear, sometimes zoned crystals with fair euhedral shape. Some crystals appear to be intermediate between the two types. The lack of any evideuce of rounding suggests that the gneiss may be meta-igneous. EXPERIMENTAL PROCEDURES General

Zircon was recovered from rock samples of -100 methods.

lb by the usual specific gravity and magnetic

E. J. CATANZAW~ and J. L. KUL~

96

Contamination during chemistry was minimized by extensive base and acid washing of al1 equipment as well as by the use of teflon box enclosures during evaporation steps. Blank determinations were always less than 0.6 ,ug Pb. Sample sizes were adjusted to yield ~100 In spiked aliquots, sample and spike ,ug so that the blank was always less than 1 per cent. size were adjusted so that the resulting Pb20s/Fb208 ratio was close to 2. In this way contami nating lead (presumably common) would have a minimal effect on the measured ratio.

Chemical procedures * A 100-600 mg sample of zircon grains is slowly sintered over a gas flame (T = 600-700°C) with 2-10 g of KHF,. The sintered material is dissolved in 0.5 N HCl and water. Final volumes are usually from 200 to 600 ml ( ~0.3 N HCl) and are held in a polyethylene bottle. The solution is weighed and split into two aliquots. The smaller aliquot (~25 per cent) ia spiked with known amounts of Pb20s, U% and Th230. The hydroxides of zirconium and the trace elements are precipitated by bubbling in NH, gas. The solution is centrifuged and the liquid is discarded. The hydroxides are dissolved in dilute HCl; t,he solution is evaporated to dryness, and the residue is dissolved in 1 N HCl. The following ion-exchange procedure refers to the spiked aliquot. The unspiked aliquot is processed in an identical manner, but only for the lead.

1. A l&cm (length) x 1 -cm (diameter) column of Dowex 1 anion exchange resin is prepared by settling the resin in dilute HCl, cleaning it by elution with 25 ml of 8 N HCl, and pretreating it by olution with 25 ml of 1 N HCl. The flow rate is controlled by a teflon plug stopcock at tho bottom of the column. 2. ‘I’hta solution is put through the column (flow rate ~1 ml/min). The lead is retained by the rosin but the uranium, thorium and contaminating elements are not. The eluate (containing the U and Th) is collected. 3. The column is eluted with 25 ml of 1 N HCI. The first 5 ml of eluate are collected and mixed with previous eluete; the remaining 20 ml of 1 N HCl eluate are discarded. 4. The lead is stripped from the column by olution with 25 ml of 8 X HCI. The eluate IH colloctcd whon the color change in tho resin (light-yellow - orange) approaches the bottom 01 of the column (CATASZAHO and GAST, 1960). 5. The 8 N oluate is ovaporated to dryness and tho residue is dissolved in 10 ml of 1 N HCI. 6. A 2 cm3 column of Dowex 1 anion exchange resin is prepared by settling the rosin in dilute HCl, cleaning it by elution with 10 ml of 8 N HCI, and pretreating it with 10 ml of 1 N HCl. 5. The solution is put through the column and the eluete is discarded. 8. ‘I’ho column is aluted with 10 ml of 1 N HCl, tho eluete being discarded, and finally with 10 ml of 8 N HCI. The 8 N HCl eluato (containing tho Pb) is collected. 9. The solution is evaporated to dryness and the residue is converted to nitrates by adding 1 ml of concentrated HNO, and redrying. The nitrate residue is dissolved in 2 ml of 2% HNO,. 10. The solution is transferred to a 3 ml centrifuge tube and the pH is adjusted to 4.5 with NH,OH. The solut.ion is heated in a water bath and PbS is precipitated by bubbling H,S through the solution. 11. Tho lead is introduced into the mass spectrometer by pipet,ting a slurry of the PbS !. solution onto a filament in the source assembly. I.‘ranium

re.coven/

1. The solution containing the uranium and thorium (2, above) is evaporated to dryness and the residue is dissolved in 20 ml of 6 N HCl. 2. The large resin column, from which the lead will have since been removed, is treated with 20 ml of 6 N HCI and the uranium and thorium solution is poured on. At this normality (6 N), the uranium is mtainod by the resin. The eluatr, containing the thorium, is collected. l The ion-exchange procedures used in this study worn devised by reference to the detailed studies of KRAUSE and NELSON (1960) at Oak Ridge.

Discordant

zircons

from the Lit,tln Belt, Beartoot,h

and Santa Catalina

Mountains

97

3. The column is &ted with 20 ml of 6 N HCl. The first 5 ml of eluate are collected with the previous eluate and the remaining 15 ml are discarded. 4. The uranium is recovered from the column by elution with 12 ml of 1 N H&W,. The first 5 ml of eluate, which consist primarily of 6 N HCl trapped in the resin and the glass tubing of the stoppered arm, are discarded. The remaining H,SO, cluate, containing the uranium, is collected and evaporated to dryness. 5. The sulphate residue is dissolved in 1 ml of concentrated HXO,, transfcrrcd to a 2 ml beaker, and redried. 6. The uranium is int,roduced into the mass spectrometer by dissolving the rrtsiduc: in enc. or two drops of loo,& HNO, and pipetting the solution onto a filament in the source assembly. Thorium

recovery

1. The solution containing the thorium (2, above) is evaporated to tlryness and tho residue is dissolved in 15 ml of -10 N HCl. 2. The large resin column, from which the uranium will have since been rcmovrd, is treated with 15 ml of 10 N HCl, and the thorium solution is poured on. Zirconium is hcl11 by tho rosin and the eluate, containing the thorium, is collected. 3. The column is eluted with 5 ml of -10 N HCl and the eluate is collected with the previous eluate. The column is now discarded. 4. The solution is evaporated to dryness and the residue is dissolved in 10 ml of 6 N HCI. 5. A 2 ml column is filled with Dowex 50 cation exchange resin and pretrrat,ed by elution with 10 ml of 6 N HCl. The thorium is retained hy the resin anti the 6. The solution is put through the column. eluate is discarded. 7. The column is eluted with 10 ml of 6 N HCl to insure complete removal of thtl potassium. The eluate is discarded. 8. The thorium is recovered from the column by elut,ion with 10 ml of l/2 molar oxalic acid. 9. A few drops of HClO, are added to the oxalic acid eluate and the solution is evaporated to dryness. The residue is dissolved in 1 ml of concentrated HNO,, transferred to a 2 ml beaker, and redried. 10. The thorium is introduced into the mass spectrometer by -dissolving the residue in one! or two drops of 1Oqa HNO, and pipetting the solution onto a filament in the source assembly. The procedure for monazite analysis is identical to the zircon procedure except that, after dissolution of the sample in 1 N HCl, a small amount of purified AlCl, solution was added to each aliquot to act as a carrier in the hydroxide precipitation step. The correct use of isotope dilut,ion techniques necessitatrs quantitative decomposition of the sample as well as quantitative recovery in all steps prior to spiking. Also, there must he complete equilibrium between the spike and the dissolved sample. In order to test the procedure In these experiments the spikes described above, two pre-spike experiments were performed. were added to the samples prior to the fusions. The lead concentrations were calculated using Table

1. Adclition

of spike before Concentrations

and after fusion (ppm)

Sample

SG* SGt 7* 7t

1:

Pb

Th

2088 2098 1332 1332

612 61s 401 403

--

* Normal procedure. t Spiked before fusion.

465 445

98

E. J. CATANWHO and J. L. KVLY

the isotopic composition values obtained in the original determinations. If any uranium, thorium or lead is lost during the fusion and dissolution of samples, the concentration values obtained in the pre-spike tests would be higher than those obtained by the usual prodedure. Table 1 compares the results of the two procedures as determined for two zircons. These results suggest that the analytical procedure used in this study gives quantitative recovery of all thret? elements prior to spiking. The difference in the thorium results is slightly outside the estimated experimental error, but since the prespike result is low the discrepancy cannot be explained hy loss in the normal procedure. &lass spectrometrg Isotopic measurements were made on a stainless steel, direction focusing, 60’ sector field, 6-in. radius of curvat,urc mass spectrometer. Doflning slits of 0.004 in. and a collector slit of 0.010 in. were used. The surface ionization source used in similar to that described by ALDRICH et al. (1953), except that the filament is mounted by means of a micro-vise rather than by welding. In order to avoid cross contamination, ion source parts exposed to the filament were washed in hot 50% HNO, between runs. The ion detection system oonsisted of a 12-stage electron multi., plier in conjunction with a vibrating reed electrometer. A detailed description of the instrument can be found in MILLER (1960). The accuracy of the isotopic measurements is limited by a number of processes which can cause discrimination. Measurements on gravimetrically prepared isotopic standards* show no net systematic discrimination within the precision of this instrument. The precision of the Pb20’/Pb20s and Pb2es/Pb2w ratios in terms of the standard error on the mean were -t0.25 and & 0.50 per cent, respectively. Potasuium-argo~~age determina&ns The radiogenic argon was extracted by a direct fusion technique and measured by isotope dilution mass spectrometry (LONG and KULP, 1962). The reproducibility and absolute calibration of the argon determination was better than 1 per cent. The potassium content of the minerals WM determined by isotope dilution, using a K*l spike. The reproducibility is about 1 per cent and the analytical ages of the Precambrian samples are believed to be accurate to within 2 per cent (KVLP et cd., 19633.

All of t.he analytical results and ages obtained in this study are summarized in Tables 2, 3, 4 and 5. The standard error on the mean for PbZ07/Pb2’J8 and Pbew/ Pb206 ratios is less than 0.5 per cent in all cases. The standard error on the mean for the individual Pbz04/Pbz06 ratios is given in the tables. The possible errors in spike calibration, sample dissolution, weighing procedures and mass spectrometer ratio determinations, suggest’ a conservative limiting error of 2 per cent for the reported concentrations. Given in Table 2 are the analytical data for the zircon and monnzite samples from each of the three localities. The concentrations of U, Th and Pb are given in parts per million. The isotopic compositions are given relative to Pb206 -= 100.00. The isotopic ages, rock type and concentration of common lead for all samples of zircon and monazite summarized in Table 3. Errors on the U-Pb and Th-Pb ages were calculated on the basis of a possible 3 per cent error in the ratios. Errors on the Pb207/Pb20‘J ages were calculated on the basis of the limiting mass spectrometer errors and consideration of errors in the common lead corrections. The potassium and argon determinations are given in Table 4. * We are grateful to Dr. GEORGE TILTON of the Geophysical Lab., Carnegie Institution, Washington for supplying URwith these standards.

Discordant

zircons from the Little Belt, Beartooth Table 2.

Analytical

and Senta Catalina Mountains

99

data on zircon and monazite Pb atom abundance relative t,o Pb206 = 100.00

Sample Number

Uranium

Thorium

A. Little Belt Mount&as, Montana 11 2227 1421 261 13 549 217 138 14 1417 276 216 14M* 2134 88400 8020 15 375 110 26 361 101 113 27 590 154 138 31 1047 338 176 34 553 159 115 41 710 346 123 Beartooth Mou&zins, MontarLa 2088 612 1330 280 1662 1258 536 1332 872 430

B. SG GL 1 4 : 7

1548 1600 1332

Mountairrs, 299

206

207

208

0.437 & 0.004 0.144 & 0.001 0.152 i 0.001 0.320 i 0.003 0.253 k 0.002 0.125 & 0.001 0.190 f 0.003 0.330 & 0.002 0.096 f 0.001 0.159 * 0.001

100 100 100 100 100 100 100 100 100 100

15.98 12.98 13.70 16.07 18.18 17.18 17.51 15.31 12.21 12.94

“5.81 18.27 14.47 11.32 19.40 13.63 16.57 24.47 11.64 16.97

100 100 100 100 100 100 100

20.46 25.42 23.99 19.79 23.20 23.39 19.39

“4.26 50.20 36.40 14.36 5.24 10.18 22.89

100

10.69

14.54

l 0.0002

507 521 401

0.105 0.683 0.447 0.049 0.045 0.058 0.153

Arizona 34.1

0.143

i_ 0.0006

834 238 465

C. Santa Catalina 100 990

204

Lead

+ f & i_ & &

0.002 0.0005 0.001 0~0003 0.001 0.001

* Monazitt: Table 3. Isotopic

ages of zircon and monazite

samples*

Kock Sample A.

type

_

Little

Belt Mountains,

Montana

11

Migmatite

20.2

560

&

20

830

*

35

1660

&

40

41

Gneiss

8.0

910

*

30

1210

*

35

1800

&

40

500

31

Gneiss

15.5

810

&

25

1130

&

35

1810

+

45

950

34

Augen

5.1

1140

*

35

1400

+

40

1820

*

40

1080

13

Migmatite

5.5

1290

+

40

1540

*

45

1910

*

40

1550

14Mt

Migmatite

1.8

1860

_C 40

1890

*

40

1920

f

30

1790

14

Migmatite

7.7

830

&

1190

+

35

1940

*

40

1260

Gneiss

25

15

Migmatite

11.7

1390

*

40

1830

*

50

2380

h

46

27

Migmatite

9.2

1170

*

35

1670

f

50

2390

*

45

26

Migmatite

6.0

1570

_+ 4;

1980

i

60

2450

+

40

B.

c.

Beartooth

Mountains,

25

1400

*

40

2580

+

50

40

1970

&

60

2650

&

50

2350

Gneiss

17.8

1280

+

40

1930

If

60

2730

*

50

1260

Gneiss

4.6

1350

+

40

2020

f

60

2800

&

50

Amphibolito

2.3

1600

*

50

2180

&

65

2800

i

50

1050

Migmatite

2.0 2.8

1630

+

50

2360

+

70

3080

&

50

1530

1660

*

50

2380

5

70

3080

*

50

870

7.6

200

&

6

330

*

10

1390

&

30

200

Granite

1

Granite

SG

Granite

4 5 6

Granite Catalina

100 . ~~~~~

24.8

+

1

Gneias

Gneiss

constants,

t Monazite.

770 &

+

6

‘blountai?l%, Arizona

Gneiss

y-1.

40

Montana 1400

Granite

*

1500

6.7

GL

Santa

320

~“38:

1

=

1.54

x

lo-10

y-‘;

u=s:

A =

9.72

Y

lo-"'

y-l;

Thz3*:

2 =

4.88

x

lo-*’

Table 4. K-Ar

_-

SaDlple Number -_-__

Rock type -_--..

andytical

tlltta and isotopic ages*

.___.

A.

Little Bell Mountaina, Montaruc K-1H tlaramide Porphyry Pinto Mcta-Diorite K-2H K-3H Pinto hleta-Uiorit0 K-413 Pinto Meta-Diorite K-4H Pinto Meta-Dioritw K-5B Pinto Meta-Diorite K-5H Pinto Mnta-Dioritv K-1OB Pinto iMota-Diorite K-1OH Pinto Meta-Diorite9 Migmatitn 10H Migmtttit<> 11H 14B Migmetite 15H Migmatitu Augen Unoiss 34H

0.08 1.03 ‘.Q!> 1.31 4.27 1.43 3.20 1.38 1.46 2.12 5.74 1.20 2.27

1.43 8.61 a.70 24.2 11.7 4i.i 17.4 35.0 16.2 16.6 21.4 52.2 11.6 “3.1

8.52

1.11

2.86

120 .f: III 1465 1420 1380 1480 I720 18’0 1700 1780 1740 1610 1500 1780 1620

43 85 84 Hi ‘35 w Hi Y8 !)5 8X x4 ‘IQ Q8 9i

t%. Bmrtooth Mountuina, Montana 4H Amphibolite C. San&aCatalina Moun&na, 100 Mu GneisR * Decay constants. t H = hornblende.

Arizom

__-

?06/204

207/204 .____-__.

Little Belt Moztntaisw, Mon-tanu Galene- 1 16.92 f 0.1’72 Microcline-1 15.50 f 0.28 Micro&m-2 15.85 f 0.08 Microcline-3 15.38 f 0.06 Beartooth Mountains, Montanrc Microcline-4 14.11 5 0.22 Microcline-5

-~

14.31 f

Region

32

:1

K”‘: Ae = 0.584 x lo-‘* y-1; 1~ = 4.72 y 10-l” y-l. H = biotite. Mu = muscovita.

Tablo 5. Common Sample

76

0.16

_________~____

Little Belt Mountains (1900) Little Belt Mountains (2400) Beartooth Mountains Catalina Mountain8

15.33 15.13 15.26 14.99

f -& -& +

lead data 208/204

0.16 0.27 0.09 0.06

36.75 3560 36.30 34.76

& i_ & *

0.37 0.64 0.35 0.13

14.99 h 0.22

33.86 4: 0.45

16.08 $- 0.19

33.77 *

206/204 15.20 14.60 14.22 16.00

~__

800 1700 1550 1650

f * & *

150 250 150 150

lnvestigatom

Present work Preesentwork Preeent. work Present work Catsnzaro and Gast (1960) Cat-ro and Gest (1980)

0.50

‘“07/204 -. __.____~~ 15.10 15.17 1504 15.48

Model age (1n.y.)

?08/204 -_ _~~~______._. 34.50 34.18 33.72 X5.80

. .._.~~ ___.

Common lead corrfclion Original (common) lead, i.e. lead incorporated in a mineral during its crystallization is identified by the presence of the non-radiogenic Pba04 isotope. _A certain amount of common lead may be inadvertently introduced during the complicated chemical analyses of zircons, but as the blank studies indicate, this is generally less than 1 per cent and should not be significant. Thus, despite unfavorable size and charge conditions, it must be concluded from the results given in Table 3 that the zircons incorporated small amounts of common lead during their crystallization, or possibly at some other time during their history.

Discordant zircons from the Little Belt, Beartooth and Santa Catalina Mountains

101

Since the isotopic composition of lead in the earth is continuously changing with time, the isotopic ratios of the incorporated common lead cannot be known with complete certainty. The composition is a function of the age of the lead (i.e. the time of crystallization) and the previous history of the lead (i.e. the U/Pb and Th/Pb ratios of all the previous environments in which the lead accumulated). It is virtually impossible to know the environmental history of any particular sample of lead, and attempts to describe the time-dependent variations are usually based on models which effectively limit environmental history. The fundamental concepts which have been used in all general treatments of time-dependent lead isotope variations were independently suggested by GERLING (1942), HOLMES (1946) and HOUTERMANS (1947). CAT~NZARO and GAST (1960) used a simple Gerling-Holmes-Houtermans type model to date microgram quantities of lead extracted from pegmatitic potash feldspars. Their resulting model ages were in good agreement with independently determined true ages. Thus, although the assumptions of the simple model are almost certainly not followed by the majority of lead samples, variations from the model appear to be small enough so that in general, good estimates of the isotopic composition of the rock lead of all ages may be obtained from the simple model. The best approach to obtain the isotopic composition of the common lead in a zircon is to measure the isotopic composition of lead in cogenetic, non-uraniumbearing minerals. If such measurements are not available, it would appear reasonable to use the isotopic composition derived from a simple common lead model. Both methods have been used in this study. The model used is identical to that used by CATANZARO and GAST (1960). Table 5 summarizes the results of isotopic analyses of lead from one sample of galena and five pegmatitic microclines from the Montana areas, as well as the isotopic compositions of the common lead corrections used. In the Little Belt Mountains, the ore body is post-porphyry, and therefore, probably post-Cretaceous and the galena lead is obviously anomalous. This type of anomaly, consisting of lead with a lower radiogenic lead content than that expected from the model, is rather rare and may be the result of remobilization of older lead ore. Stringers of remobilized older lead have been found in the Coeur d’illene district of Idaho (LOKG et al., 1960). The authors attribute the remobilization to heating by adjacent Laramide porphyry intrusives and suggest, by extension, that larger quantities of older primary lead ore might be redeposited without significant contamination by contemporary rock lead. In the present area the 800 m.y. model age is not correlated with any event and this would suggest that the present lead ore may be a mixture of remobilized lead from old ore deposits and younger rock leads. The microclines were not analysed for uranium and it is possible that they contain some radiogenic lead. However, the reasonable similarity between the model ages and the probable true age (1900 m.y.) made it seem plausible to accept a common lead correction derivable from the simple model previously referred to. For the younger group of zircons an age of 1900 m.y. was picked, along with a (U238/Pb204), ratio equal to 8.5. The choice of the 8-5 growth curve was dictated by the fact that, when plotted on a Pb207/Pb204 vs. Pb206/Pb204 graph, all four of the measured common leads fell on or near this growth curve. For the older zircons in this area

102

$1. *J. C'AT_WZAKO

and J. L. KULP

( Pbz07/Pbzos z 2400 m.y.), the common lead was again chosen from the model, with 1 = 2400 and (U238/Pb204), = 9. Empirical results of hundreds of common lead isotope analyses indicate that (U23*/Pb204), = 9 is t,he best choice when no independent evidence is available. Two determinations of feldspar lead from the Beartooth Mountains are available from the work of CATANZARO and GAST (1960). Since the model age derived from these leads equals the apparent rock age, the correction employed was a simple average of these determinations. No feldspar lead data were available from the Santa Catalina Mountains area ‘I’ablr 6.

‘i%p t?f+ct of Itsing modern common lead as a correction for sample No. 11 ..-_ .--

Common Pb used as correction

Ages (m.y.) [;23s_Ph206

u'd35Lpb207

560 550

830 810

ph207_~$206

_._ .._ _~~

1900 m.y. (I/, = 8.5) Modern (see text)

1660 1640

so the isotopic composition on the common lead correction was chosen from the model, with t = 1650 and (V3s/Pb2’J4), = 9. The common lead correction is small for these zircons since they have great age and contain relatively small amounts of common lead. Only if the common lead was extremely anomalous, and the feldspar leads appear to preclude this, would the errors be significant. Table 6 shows the effect, on the isotopic ages of using modern common lead as a correction from the zircon from the Little Belt Mountains with the highest common lead content (e.g. sample No. 11). The isotopic composition of modern common lead was taken as that found in an oceanic manganese nodule: Pb206/Pb2’J4 = 18.91, -Pb2”‘/Pb2”4 = 15.69, Pb20*/Pb 204 = 39.34 (CATANZARO and GAST, 1960). The most strongly affected age (U235/Pb207) is changed by less than 3 per cent and it is, therefore, evident that the common lead correction does not directly enter into the basic problem of discordance in t,hese zircons.

If the isotopic ages derived from the Pb206/U23” and Pb207/U235 ratios in it mineral differ by more t.han 5 per cent the result are said to be discordant. Such a difference is non-analyt,ical and must be attributed to physical or geochemical alteration of the uranium-lead system in the mineral. Although zircon samples generally yield discordant results, certain speoimens have been found which are concordant. The degree of discordance appears to be related to the structure and composition of zircons from different areas and their post-crystallization history. Discordant zircon samples invariably yield the same age pattern: U238-Pb206 age %.:U235-Pb207 age < Pb207-Pb206 age. The Thgs2Pbzo” age may be greater or less than the U236-Pb206 age but is alwaye less than the Pb207--Pb206 age. Mathematically. this pattern of discordant uranium-lead ages

Discordant

zircons from the Little

Belt,

Beartooth

and Santa Catalina

Mountains

103

can result from either addition of uranium, loss of lead or loss of one or more of the mechanism is intermediate decay products of the U z3* chain. The last-mentioned based primarily on the possibility of radon and/or radium loss. The isotopes of these elements found in the U238 chain (Rn222 and Ra2z6) have significantly greater half-lives thanthose of the analogous isotopes in the U235 chain (Rn2ig and Ra223). Thus, migration of these elements out of a mineral or to sites from which subsequent members of the decay chain might be more easily lost would result in isotopic ages with the zircon-type discordance. Although this process may explain the discordances of some uranium-bearing minerals (COBB and KULP, 1961), there are several reasons for considering it unlikely in the case of zircons. In general, K-Ar and Rb-Sr ages on cogenetic mica agree most closely with the Pb207-Pb206 age of the zircons (ALDRICH et al., 1958). If preferential loss of a nuclide in the U238 chain were the dominant process causing zircon discordances, the U235-Pb207 age would be most nearly correct. Also, where multiple samples of discordant zircons from the same area have been analysed the Pb207-Pb206 ages are relatively constant), while the U2351958; SILVER and DEUTSCH, 1961; TILTOX, 1961). Pb20’ ages vary (Kouvo, Finally, concordia plots (see below) of discordant zircon results never follow U23s daughter loss patterns. Of the two remaining processes which could give rise to the age pattern shown by discordant zircons, uranium addition and bulk lead loss, evidence from leaching studies (TILTON, 1956) and U/Th ratio studies (TILTON et al., 1957) suggests that lead loss is the more likely mechanism. In many areas the Th232-Pb208 age of discordant zircons is invariably the lowest, no matter what the Th/U ratio is in the mineral. Thus, if parent addition is the cause of the discordances it would require a regulating mechanism by which the uranium and thorium would be added in proportion to the relative amounts already present in the mineral (TILTON et al.: 1957). A sample from Tory Hill, Ontario gives the following isotopic ages: U23sU235-Pb207 == 1050 m.y., Pb207-Pb206 = 1090 m.y. and Pbzo6 = 1030 m.y., Th232-Pb208 = 390 m.y. (TILTON, 1956). If recent parent addition were the cause of this discordance it would require the addition of material with a Th/U ratio of -9. Since the highest Th/U ratio noted by HVRLEY and FAIRBAIRN (1!)57) in On their study of more than 60 zircons was 4.3, such an addition seems unlikely. the other hand, the lead-loss hypothesis would only require that the thorium and uranium be inhomogeneously distributed in such a way that the Pb208 formed by thorium decay would be located in a position from which it could be preferentially removed from the mineral. That this is indeed the case is indicated by Trr.ros’s (1956) acid leach study of this sample which showed that the soluble portion of the lead was highly enriched in Pb20* with respect to the bulk lead in the sample. Although the basic cause of zircon discordance is thus generally considered to be bulk loss of radiogenic lead, the exact mechanism of this loss is not clear. Auy proposed hypothesis must account for the following empirical observations: 1. Multiple samples of discordant zircons of the same age and geologic history appear to be linearly related when the results are plotted on a graph of Pb206/U238 vs. Pb207/U235 atomic ratios (Kouvo, 1958; TILTO~-, 1960; SILVER and DE~JTSCH, 1961).

IX.

101

J.

L’. In all cases of discordant

CATANURCI

and

J. L. KULP

zircons there appears to be an excess loss of PbZ”:

relative to Pbzo8. 3. Lead loss from zircon may occur without significant argon loss from mica in the same rock (ALDBICH et al., 1958). 4. Reheating or metamorphism sufficient to cause complete loss of argon from biotite is not a sufficient condition for lead loss from zircons (TILTON el al., 1958). ;T. Recent exposure to ground water is not a sufficient condition for significant lead loss since a few concordant zircons exist and all have been near the surface prior to collection. 6. Extensive metamictization is not a sufficient condition for significant lead loss (e.g. Wichita Mountain sample, TILTON et al., 1957). Hypotheses

to account for loss of radiogenic lead

Mechanisms which may account for zircon discordances have been developed for discordant uranium-lead ages in general. Many uranium-bearing minerals exhibit the same discordancy pattern and regularities shown by zircons. There have been three basic hypotheses suggested to account for the observed relationships; (1) episodic loss of bulk radiogenic lead, (2) continuous diffusion of bulk radiogenic lead, and (3) fractionation of lead isotopes plus recent removal. the theoretical Episodic lead loss. WETHERILL (1956 a, b) first presented construction for the episodic loss concept. On a concordia diagram (Fig. 9), i.e. curve for concordant ages plotting Pbsos/U298 vs. Pbso7/U*36 atomic ratios, the partial loss of bulk radiogenic lead from the zircons of a rock at a point in time will yield discordant isotopic ages on each sample analysed. The results from each sample will fall on a chord connecting the initial age of zircon formation with the time of the event which caused the lead loss. The location of the point on the chord will be dependent only on the percentage lead lost from the sample. In the example (Fig. 9) the chord 2800400 is used. If the episode of bulk lead loss occurred recently and no isotopic fractionation is involved the chord would pass through the origin. If two episodes of bulk lead loss occurred, one at 2000 m.y. ago and the other at 600 m.y. ago, then all points for discordant zircon samples would fall within a triangle 2800-2000-600 m.y. in Fig. 9. Such a multiple event historq appears to have altered the pitchblende in the Lake Athabasca region (ECKELMANK and KELP, 1956). The concept of episodic loss of bulk radiogenic lead does not depend on the actual process by which the lead loss occurs. It must, however, remove significant quantities of daughter lead isotopes in a relatively short time interval, i.e. l-100 m.y. on this time scale. Two probable processes for bulk lead loss are: (1) diffusion removal of radiogenie lead during high grade metamorphism (high temperature event), (2) leaching of lead by ground water. The high temperature process has been clearly demonstrated in the Little Belt Mountain area discussed below and may be expected under conditions of high grade regional metamorphism. There are, however, areas such as the central and southern Appalachians (TILTON, WETHERILL, DAVIS and HOPSON, 1958; DAVIY, TILTOS and WETHERILL, 1962) where a later metamorphism which was sufficient

Discordant zircons from the Little Belt, Beartooth and Santa Cat,alinaMountains

105

to reset the K-Ar system did not cause significant radiogenic lead loss from zircon. On the other hand it is common to find discordant zircons (and hence lead loss) in areas that have not been subjected to a subsequent metamorphic event, the absence of such an event being demonstrated by the existence of old K-Ar ages in the same rock. Thus if episodic lead loss is the general explanation for discordant zircon ages, a mechanism of lead removal must also be operable at low temperature. If exposure to ground water in the surface zone is the mechanism, then radiogenic lead loss should be universal. The extent of loss, however, would depend on the local

Pb2” u 235

9. Concordia diagram showing the theoretical loci of discordant isotopic ages for various lead loss processes.

Fig.

ground water conditions, i.e. flow rate, chemical composition and temperature. Since the stability of the lead ions in the disordered zircon lattices will vary widely, the initial exposure to circulating ground water should yield the most efficient removal. If a rock had been held at depth until recently, the results of a zircon system should fall on a straight line in the concordia plot which passes through the origin. (It is assumed throughout this section that no isotopic fractionation occurs in the leaching process.) If it had been exposed to ground water at an earlier time in its history three patterns might be found. (1) If the initial leaching were much more effective than the recent situation, a chord would result which would pass through the concordia curve at the time of first leaching, i.e. the episode. Even if a basement rock were brought to the surface say 200 m.y. ago and remained essentially in the same position to the present, it might be expected that most of the loss (all of the readily available lead) would have occurred close to 200 m.y. ago. This would give a pattern to the zircon results indicative of an episode at 200 m.y. ago. (2) If the leaching was far more effective at the present (second period), the straight line would pass through the origin. (3) If significant amounts of leaching occurred at both times, the pattern of zircon results would fall in the triangle

IOti

E.

J.

UATANZAROand J. L. KUIZ

bounded by the time of formation of the zircon, the time of first exposure to ground water and the origin. TILTON (1960) proposed that the lead loss may be Continuous dijfusion,. governed entirely by continuous diffusion over the history of the mineral. The rate of this process would be governed by a constant diffusion coefficient, D, (for a given sample), the effective spherical radius of the mineral grains, o, and the TILTOK showed that for each true age, substitution of concentration gradient. various values of D/a2 into the equation gives rise to a smooth curve on a concordia plot (e.g. for 2800 m.y. mineral see Fig. 9). These curves are nearly straight lines for much of their length and begin to curve perceptibly only near the origin,

0.6 t

Pb207 "235

Fig. 10.

Diffusion curves. The d-shed lines connect points of qial D/a2 for t.ho two curves.

hence those minerals which have suffered less than 50 per cent lead loss will appear to tit a continuous diffusion curve or the episodic lead loss chord on the concordia diagram. A graphical distinction between the two interpretations can only be made when there are samples which cover a wide range of lead loss. The advantage of this hypothesis is that it eliminates the necessity of postulating events at the lower ends of the chords. The disadvantage of the diffusion hypothesis is the requirement that II be constant. D must be insensitive to temperature variations and, possibly more important, it (as well as “a”) must be insensitive to the lattice disruption associated with metamictization. TILTON suggested that the diffusion may be controlled by temperature-independent vacancies but HOLLAND and GOTTFRIED (1955) have shown that the number of such vacancies caused by radioactive decay is at least an order of magnitude greater than the number attributable to improper crystallization so that the rate of diffusion would have to be a function of the metamictization and would increase with time. Figure 10demonstrates the relations derived from continuous diffusion lead loss

Discordant

zircons from the Lit’tle Belt, Beartooth

and Santa Catalina Mountains

107

in the case where two “events” have occurred. Assume that uranium-bearing minerals formed at 2800 m.y. and at 1800 m.y. It then follows: (a) if the 1800 m.y. event did not affect the 2800 m.y. old minerals each of the discordant results must lie exactly on the diffusion curve for its particular true age and (b) if the 1800 m.y. event caused partial lead loss in some of the older minerals, all of the points must lie on or between the two diffusion curves. (This situation, of course, is a combination of diffusion lead loss and episodic lead loss.) The dashed lines indicate the loci of older zircons, with specified D/a2, values which lost lead during the 1800 m.y. event. Empirical evidence may occasionally preclude continuous diffusion as the sole explanation for a suite of discordant zircons. SILVER (personal communication) has found that the discordant zircons from the Marble Mountain granite show a linear relationship extending well into the region of a concordia plot in which the diffusion curves are non-linear. Further, the degree of discordance in this set of samples is proportional to uranium content and hence degree of metamictization. Tt seems unlikely that D would be independent of the structural state of the mineral. Fractionation of lead isotopes. In a study of the discordant uranium-lead ages of old monazites and uraninites from Rhodesia, Madagascar and Manitoba, AH~~ENS (1955a, b) noted that the results were linearly related when plotted on a graph of log t (U238-Pb206) vs. t (U 235-Pb207). He concluded that the regularity of the results suggested that the lead loss was controlled by physical processes. RI-SSF:LL and &\HRENS(1957), in a study of discordant uranium minerals of a number of ages, showed the usual linear relationship between discordant minerals of the same age, hut further noted that the hypothetical times of lead loss were apparently related to the true ages of the minerals. That is, suites of minerals with probably true ages of 2680 , 2020 and 1800 m.y. appeared to have suffered lead loss at t’imcs of 500, 300 and 150 m.y., respectively. They concluded that such a relationship was highly improbable if a chemical process was involved and suggested that the results supported AHRENS’ (1955b) original contention that a physical process which favoured removal of Pbzo7 relative to Pbzo6 was involved. The suggested mechanism is that members of the U238 and U235 decay series are made more available to leaching due to recoil following alpha emission. The loss in each series is assumed to be proportional to the sum of the recoil distances. RTXSELL and AHRENS calculated a fractionation of 4 per cent in favour of Pbzo7 for immediate loss of each isotope, 11 per cent if only the recoil of the gaseous emanations are involved, and 30 per cent if only nuclides above the emanation are lost when they decay to the emanations. The 4 per cent fractionation would represent the average effect on the bulk radiogenic lead without regard to the efficiency of special steps. It is hard to see how the recoil of the gaseous emanations can have any effect on the ultimate leachability of the end product lead (11 per cent effect). The 30 per cent effect may be present since this would determine the relatively efficiency by which Rnzz2 (radon) and Kn21s (actinon) are injected into microfissures. Although the 4 per cent factor might be independent of mineral type, degree of fissuring or metamictization (unless annealing is involved), the 30 per cent factor should vary with the character and extent of the microfissures which,

i!:.I. CATANZARO

103

mxi

J. L. Kvr,r

in turn, should vary with the age of the mineral. The ratio of Pb20~/I”b206 in the leached lead would depend on the relative importance of these and perhaps other Nevertheless, if the lead is removed recently from :k fractionation processes. homogeneous population of zircons, and the overall fractionation is the same for all samples of the population the analyt’ieal results would fall on a curve such as is shown in Fig. 11. It is seen that the upper portion of these curves are nearly straight and would extrapolate to an intercept with the concordia curve above the origin. For a fixed per cent fractionation this intercept would be farther from the origin as the initial time of f’ormation of the mineral increased. Thus, for 16 per

RAOIWENIC LEAD RENOVED

235 Pig. 11. Discordia

2606 and 1660 m.y. ago 16 per cent fractionation forming Pb’Or removal. Solid curves in&~ete ideal locus for con&ant fractionation factor. Dashed linee show ertrapohted CUTV~S

for miIWr8h3 formed at 3000,

dg

straight linea from upper, nearly linear, portion of curvea.

overall fractionation the time of formation-intercept pairs would be about 3000-400 m.y., 2000-300 m.y. and 1000-200 m.y. For 4 per cent overall fraction the extrapolation of the upper half of the fractionation loss curve would yield the approximate intercepts 3000-160 m.y., 2000-120 m.y. and 1000-80 m.y. If the These are the intercepts to be expected if the leaching occurred recently. mineral was subjected to an earlier period of leaching and negligible leaching is occurring now, another similar set of curves would be obtained whose intercepts, extrapolated from the upper straight line portions, would lie above the actua1 times of lead lose. If an older period of leaching was followed by significant present day leaching, the analytioal results would fall within the triangular area bounded by the two fractionation curves. Even homogeneous population of zircons carry variable uranium content, have variable degrees of metamiotization and, therefore, variable concentration of microfissures. Thus the fractionation fa&ors should vary from sample to sample causing deviation from the ideal curves of Fig. 11. If the most metemict samples

cent

Discordant

zircons from the Little Belt, Beartooth

and Santa Catalina Mountains

109

which have the greatest lead loss have also the largest contribution of the 30 per cent factor, straight-line extrapolation of Fig. 11 would be closer to the actual curve at higher percentage lead loss. The zircon system in the Marble Mountain granodiorite investigated by L. T. SILVER (personal communication) shows strict linearity to 72 per cent loss of lead and an extrapolated chord with intercepts 1470-180. To obtain a 180 m.y. intercept for a 1470 m.y. time of formation, about 7 per cent overall fractionation is required. For such fractionation the fractionation curves would depart significantly from linearity at 80 per cent. As can be seen from the 2000 m.y. discordia curve of Fig. 11, it is difficult to separate these effects unless samples are available over a great range of lead loss. Thus if the actual time of formation were unknown, a straight line could be drawn through the 20, 40 and 60 per cent lead loss points on the 2000 m.y. curve. The departure at 60 per cent from the 2000 m.y. fractionation curve can only be detected if the time of formation is taken as 2000 m.y. Nevertheless, the Marble Mountain data appear to argue against fractionation of isotopes during lead loss. Tests of the hypotheses To recapitulate, the processes postulated for the removal of lead from zircons are diffusion and leaching of metamict zircons by ground water. During high temperature (i.e. metamorphic) events, episodic lead loss by short-term diffusion seems established. However, the processes involved in the removal of lead under low-temperature conditions is less certain. The hypothesized processes will yield certain patterns on concordia diagrams. Continuous diffusion will yield a smooth curve which is almost linear in its upper portion. Episodic ground water leaching, i.e. leaching only during a short period of time, will yield a perfectly straight line if no isotopic fractionation occurs, and a slightly curved line if systematic isotopic fractionation occurs. If ordinary ground water is capable of leaching lead from metamict zircons then, in general, points should fall within narrow triangular areas on a concordia plot. However, if the rock has only recently been exposed to ground water, the patterns would be identical to those of episodic ground water leaching. Unfortunately, unless there is a considerable spread in the amounts of lead loss among samples, all of the above processes and situations could result in apparently linear plots. If exact linearity is found over a wide range, episodic loss is strongly indicated. If no metamorphism is apparent “activated” ground water would be the most likely agent for lead removal. On the other hand, if the RussellAhrens relationship appears to hold, in general, for multiple zircon sample studies from different areas, continuous diffusion or regular fractionation plus recent ground water leaching would be indicated. Fortunately, if the results show some linearity on a concordia plot, even if they are insufficient for a well-substantiated choice of lead-loss process, a reasonable estimation of the time of formation can be made. Under the episodic lead loss hypothesis or the fractionation plus recent removal hypothesis, the predicted time of formation will be identical. Under the diffusion hypothesis the age will be about 100 m.y. older at 2000 m.y. and 300 m.y. older at

E. J. CATANZAFCO

110

and J. L. KULP

2800 m.y. if the lead loss lies in the 30-60 per cent range. If zircons with only lo-20 per cent lead loss can be found in the suite, the time of formation obtained from a diffusion curve fitted to the data will not differ greatly from that calculated Therefore, with sufficient effort and enough samples from the other hypotheses. it should be possible to accurately estimate the time of formation of any homngeneous zircon suite providing they have not been subject to a subsequent metamorphism. The absence of a second metamorphism can be easily identified b) K-Ar de~rminations on associated micas. The lower intercept of a chord drawn through a discordant zircon suite has no For episodic lead loss the lower age significance for the last two meohanisma. intercept would indicate the time of dteration. If the analytical data for a suite of zircon samples does not fall on a straight line then multiple episodes of zircon formation and alteration are inferred. In such cases the oldest Pbao7JPb206 age will set a minimum time of crystallization.

Samples of hornblende and biotite were taken from the Pinto metadiorite in the Little Belt Mountains at various distances from the contact of a 45-ft thick granite porphyry dike. The dike was emplaced about 120 m.y. ago in mid-Cretaceous time. The results (Table 7) show that there is no detectable effect on either Table 7. Effect of 45 ft porphyry dike on K-Ar ages of adjaoent basement rock (pinto rne~io~~) Sample Number -___K-ZH-I Kc-BH-II K-4H K-4B K-5H K-5B K-IOH K-1OB

Distance from Intrusive O-4 in. 4-S in. 5 ft 9 ft 300 ft,

Mineral

K-Ax Age (m.y.)

Ho~blend~ Hornblende Hornblende Biotite Hornblende Biotite Hornblende Biotite

1465 1420 1480 1380 1820 1720 1780 1700

biotite or hornblende at 9 ft from the contact and that at 5 ft these minerals may have lost about 15 per cent of their argon. Even in the first foot, the hornblende had not lost more than 20 per cent of its inherited argon. HART (1961) studied the analogous effect of late Cretaceous stocks in the Front Range. Biotite in the basement rock around the Eldora stook suffered oomplete argon loss out to about 300 ft from the oontact. The effect on hornblende extended only to about 10 ft. The quantitative difference between Hart’s results and the present results must be attributed to the greater heat content of the larger stocks as compared @ that of the small dike (46 ft thick) in the Little Belt Mountains. In any aase, the lack of drastic argon removal from biotite suggests that the thermal effects on the zircons were insignificant.

Discordant zircons from the Little Belt, Beartooth and Santa Catalina Mountains

111

Little Belt Mountain area Samples from this area are located in Fig. 2 and are composed of biotite and hornblende for K-Ar determinations and of monazite and zircon for U-Pb isotopic analysis, The results are given in Tables 3 and 4. The geologic evidence has shown the existence of at least two distinct periods of metamorphism. The K-Ar ages on biotite and hornblende are minimum ages since any heating effect including prolonged deep burial will cause argon loss. The K-Ar dates on these samples away from this Cretaceous porphyry dike range from 1500 to 1720 m.y. for biotite

qpbZO’

6

“235

Fig. 12. Little Belt Mountains-best-fit

straight lines.

and 1610-1820 m.y. for hornblende with an analytical error of about *40 m.y. From this it may be concluded that the minimum age for the last metamorphism in this area is about 1800 m.y. These samples cover the entire .area and most major rock units. Some of the lower ages, down to 1500 m.y., may have been caused by a local Cretaceous temperature rise due to other hidden intrusives, or simply due to deep burial after the 1900 m.y. event. The large Cretaceous stocks of the area (Fig. 1) appear too far away to have caused the observed result. The monazite sample (14M) gave concordant U-Pb isotopic ages within the error (Table 3). These data suggest a probable age of formation of 1920 f 30 m.y. assuming the slight difference in the isotopic ages is due to lead loss. This date would appear to represent the end of the last metamorphism in the area. The minimum K-Ar dates are consistent with this conclusion. Nine zircons were analysed. They are all discordant and show a range in Pb207-Pb206 ages from 1660 to 2450 m.y. (Table 3). All of the samples with Pb207Pb206 ages in excess of the time of the last metamorphism lie in the northernmost mile of the exposure (i.e. Nos. 14, 26, 27, 15). These will be referred to as the

E. J. CATANZARO and J. I,. KULP

113

“older group”. In both the older and younger groups the fraction of lead lost increases with the uranium concentration. This relationship is most readily ascribed to an increasing ease of lead removal with increasing radiation damage. Sample No. 14 appears to lie on the border of the two groups. Figure 12 shows the zircon and monazite results plotted on a concordia diagram The zircons have been split into the older and younger groups and a straight line has been fitted to each group. (The monazite data was not included in the bestfit straight line calculation.) The chord for the older group intercepts concordia at 440 f 50 and 2630 & 60 m.y. The four samples put into the older group

+

c~+--_+-t-_.$-

7

5I

B

*

Ia

”

fSX

12

II

14

I,6

PbmT6

-F

Fig. 13. Little Belt Mountains-continuous difhsion interpretation.

(Nos. 15, 26, 27 and 14) not only form a geographical group, but were previously

considered a morphological group on the basis of the statistical width and length studies. Also, there is a direct oorrelation between lead loss and uranium content for this group. The rock containing zircon samples No. 14 carries the concordant monazite (No. 14M) so that a strong effect of the 1920 m.y. event on No. 14 zircon can be assumed. The best-fit line through Nos. 16, 26 and 27 would pass below No. 14 and would produce intercepts on aonoordia of about 2560 and 300. In this case No. 14 zircon would fall between the older and younger groups as if it were only partially reset in the 1920 m.y. event. The best-fit straight line for the younger group touohes all samples within the analytical error. The lead loss ranges from 40-80 per cent. The upper and lower intercepts of this chord are 1936 & 40 and 170 f 60 m.y. The upper intercept agrees well with the estimated age for the last metamorphism derived from the concordant monazite (1920 f 30 m.y.). The morphology of these zircons does not suggest that they were newly formed during the last metamorphio event. Therefore, it is ooncluded that theee are older zircons whioh lost all of their Iradiogenie lead about 1920 m.y. ago.

Discordant

zircons from the Little Belt, Beartooth

and Santa Catalina Mountains

113

If fractionation favoring Pb2s7 and recent lead removal is assumed, the data could be interpreted as follows. The young group show an average fractionation of about 6 per cent. The older group was formed about 2630 m.y. ago and shows an average fractionatjon of about 12 per cent assuming No. 14 was unaffected by the 1920 m.y. event. If No.14 was affkcted, the estimated time of formation would be about 2550 m.y. and fractionation of about 8 per cent would be required. If samples 26, 27 and 15 were also affected by the 1920 m.y. event, the true time of formation or earlier recrystallizat,io~l of the older group could greatly exceed 2550 m.y. Comparing Fig. 11 with Fig. 12 it is seen that under the fractionation hypothesis, sample 11 would be expected to fall below the straight line extrapolated from the upper part of the fractionation curve. Another problem with the fractionatiori hypothesis is that the area was probably exposed to ground water just preceding the deposition of the Belt series ( ~1500 m.y. ago), again in Upper Cambrian time (-500 m-y. ago), perhaps during the Laramide uplift in the late Tertiary and Quaternary. Since the degree of crystal damage was relatively minor by the Belt time, the exposure would have had little effect. If Cambrian conditions were similar to those today, the resulting lead loss might represent a mixture of the two extractions which in turn would yield an apparently high percentage fractionatioil. Thus, if some 4 per cent fractionated lead was removed 500 m.y. ago and the rest recently, the net effect would be a discordia pattern suggesting a fractionation greater than 4 per cent. On the other hand, lead removal at two different times might be expected to produce a greater scatter in the young group than is observed. Figure 13 is a concordia plot of the data with two diffusion curves. The 1920 m.y. curve was chosen because this is probably the age of the last metamorphism in the area, and the 2800 m.y. curve was arbitrarily chosen so that all of the results would fall on or between the two curves. The diGsion hypothesis would explain the results as follows: all of the zircons were initially formed or reset at some time 22800 m.y. ago. They systematically lost lead by continuous diffusion until 1920 m-y. ago. The 1920 m.y. metamorphism caused episodic loss of varying amounts of lead; Nos. 13 and 34 suffered extensive lead loss; Nos. 31 and 41 suffered moderate lead loss; and Nos. 11, 14, 15, 26 and 27 suffered little or no lead loss. After the metamorphism, all of the samples continued to lose lead by continuous diffusion. Estimates or the amounts of lead loss during the 1920 m.y. event can be obtained from the positions of the points along the lines connecting identical D/a2 values on the two curves (cf. Fig. 10). This assumes that the D/a2 value for each sample was the same before and after the event. Although the above explanation for the distribution of points is possible, it seems unnecessarily complex and there are some inconsistencies. There is no correlation between the present D/a2 values and the varying effects of the 1920 m.y. metamorphism. For example, sample 11, with the highest D/a2 value would have lost no lead during the event, while sample 13, with the lowest D/a2 value would have suffered the greatest amount of Iead loss (~85 per cent) during the metamorphism. This problem might be alleviated by assuming that the D/a2 values were changed during the metamorphism, i.e. that the sample which lost lead during the metamorphism had higher D/a2 values than the present D/u” value of

114

E. J. CATANZARO

and J. L. KULP

sample 11. Because of the high value for sample 11 this would appear unlikely. Moreover, the change in D/a2 would have to be attributed to partial recrystallization and would have to include the unlikely assumption that metamorphic recrystallization of partially metamict zircons yield better crystals than crystallization in an igneous or ultrametamorphic environment. Finally there is no correlation with rock type or geographic location which would explain the preferential recrystallization necessary to make the results entirely compatible with the diffusion hypothesis. Figure 14 shows a concordia plot of the data as it would be interpreted according to the episodic lead-loss hypothesis. The three ages chosen are 120 m.y. (the age of

Fig.

14.

Little Belt Mountains-+pisodic

lwd low interpretation.

the Laramide porphyry dike), 1920 m.y. (the age of the concordant monazite) and 2470 m.y. (an arbitrary value which yields a chord from 120 m.y. through Nos. 15, 26 and 27). Note that the 1920-120 chord fit.s the data almost as well as the 1920-170 chord of Fig. 12. Each result except sample 14, lies on one of the other of the two straight lines. The episodic lead loss hypothesis could explain the results as follows: all of the zircons were formed or reset about 2470 m.y. ago. During the 1920 m.y. event, samples 11, 13, 31, 34 and 41 lost all of their lead, sample 14 lost a large portion of its lead, and samples 15, 26 and 27 were unaffected. All of the samples then lost lead about 120 m.y. age. at the time the porphyry dike was intruded. It is seen that the episodic lead loss hypothesis fits the data quite well. The grouping of the zircons is morphologically and geographically reasonable. Since there is a direct correlation between degree of lead losa and uranium content, the

Discordant zircons from the Little Belt, Beartooth and Santa Catalina Mountains

115

varying amounts of lead loss assumed to have occurred during the 120 m.y. event can be attributed to varying degrees of metamictization, i.e. the greater the degree of metamictization, the easier the lead is removed. The different effects of the 1920 m.y. event on the different zircon suites can be explained on a geographical basis. The completely rejuvenated zircons occur south of sample 14 and the completely unaffected zircons occur north of sample 14. The uniqueness of sample 14 can be explained by the chemical data in addition to the geography. While morphologically related to the unaffected zircons (15, 26 and 27), sample I4 has a significantly higher uranium content and was undoubtedly most metamict at the time of the event. Also, the fact that the concordant monazite (14M) was separated from the same rock sample suggests that this zircon sample was very likely subjected to strong forces during the metamorphism. Under the episodic lead loss hypothesis, the 2470 m.y. age must be considered a minimum since it is possible that all of the zircons (including 15, 26 and 27) lost some lead during the 1920 m.y. event. However, since the three “old” points fall on a chord, it is possible that 2470 m.y. is close to the true age, if this mechanism is the correct one, Since the present data suggests that zircons may lose all of their radiogenic lead during a metamorphic event, it is impossible to decide, on the basis of the age data alone, whether the zircon ages of the older group reflect the original age of crystallization or a subsequent metamorphism. Without, at this point, attempting to evaluate the relative probability of the three lead loss hypotheses, certain conclusions concerning the geologic history of the area can be made. 1. At least two metamorphic events occurred in the area. The time of original zircon formation was 22470 m.y. 2. The last metamorphic event in this section of the exposed Precambrian basement in the Little Belt Mountains occurred prior to 1800 m.y. ago and most probably occurred at 1920 f 20 m.y. ago. 3. The intensity of this last event was greater in the southern. part of the Neihart exposure since all of the zircons there were reset at that time. Only those from the northernmost 1 mile of the exposure show retention of some or all of their prior accumulated lead. 4. Metamorphism can cause complete loss of accumulated radiogenic lead in zircons but under certain, presumably less intense conditions, it may cause very little loss. Such a metamorphic event as occurred in the southern part of this region, therefore, is the ideal case of episodic lead loss. From the preceding discussion it is concluded that the continuous diffusion hypothesis is more cumbersome and less probable than the others. Both the episodic lead loss and the isotopic fractionation hypotheses can fit the existing data on the Little Belt Mountain area but both require certain unique conditions. For episodic lead loss the event must have been a low temperature process that occurred about 120 m.y. ago. This might be due to unique ground water conditions related to the Laramide orogeny. It might also simply reflect the first time that these rocks had proper flow of oxygenated water to provide suitable leaching of

115. J. C’ATANZARO end .J. L. KUJ,P

116

lead. If the most available lead is readily removed, the leaching date is effect&xl> the time of the initiation of the process even if the leaching has continued until thr present. A third alternative (not strictly episodic) is that more or less uniform lea. 11t t,he Little Belt area may be reconstructed: 1. Formation of original zircon in igneous rocks in a geosynclinal pile in tllca Little Belt area or deposition of pre-existing detrital zircons in this geosynclim (22470 m.y.). 2. Major metamorphism. Perhaps Pinto diorite and granite (now augen gneiss) were intruded in the later stages of this event.. :s. Cooling period. 4. Intrusion of basaltic dikes and perhaps Pinto diorite. 5. Metamorphism (1920 m.y.) caused recrystallization of rocks in the area but did not remove lead from zircons in northerly sector. 6. Uplift and erosion. 7. Deposition of Beltian sediments (at least 1400 m.y. ago based on dates in Coeur d’Alene district, LONG, SILVERMAN and KULP, 1960). 8. Uplift and erosion. 9. Deposition of Paleozoic and Mesozoic sediments. 10. Uplift and intrusion of Laramide porphyries (120 m.y.). 11. Emplacement of ore deposits. 12. Uplift and erosion. Bearloo&

Mountains,

Honta~

In a study

of the ages of the basement rocks of the Beartooth Mountains, and K-Ar ages of micas and feldspars as well as complete isotopic U-Pb ages of a uraninite. The feldspar, musoovite and uraninite samples were from pegmatites and the biotite samples were from the country rock. Thus all of the samples probably reflect the age of the last metamorphism in the area. The results obtained by these workers suggest an age of about 2700 m.y. for the last event. More recent measurements by the K-Ar method at this laboratory confirm a date of about 2700 for the latest metamorphic event, but also suggest an earlier metamorphism in the area as early as 3100 m.y. The general location of the seven zircon samples is shown in Fig. 6 and the specific locations of the four samples from the Quad Creek area are shown in Fig. 7. All of the zircon results are discordant and the Pbzo7-Pbao6 ages range from 2580 to 3080 m.y. (Table 3). The Beartooth zircons appear to be detrital and different suites contain variable but significant quantities of zircons with overgrowths. According to ECKELMANN

(FASTetal.(19.58) obtained Rb-Sr

Discordant zircons from the Little Bdt, Bcartooth and Santa Catalina Mountains

11-i

and POLDERVAART (1957), the overgrowths were probably formed during a period of granitization which accompanied the last metamorphism (~2700 m.y.). Figure 15 shows the zircon results plotted on a coneordia diagram. Two diffusion curves are also shown (2700 and 3500 m.y.). All of the points, except GL, fall on or between the two curves. The continuous diffusion hypothesis could explain the the results (for the moment ignoring GL) as follows: all of the zircons are >3;iOO m.y. old. Until 2700 m.y. ago they lost lead by continuous diffusion. At 2700 m.y. ago, all of the zircons suffered episodic loss of varying amounts of lead and/or

3 4.

,’

--k----4’

2

t

’

5

t

I

/

a

9

ro

IS

9

13

11 1

Pbm7 pFig.

15.

Bearboth

Mountains--+ontinuous dii’ksion interpretation.

After the 2700 m.y. event, the gained young zircon material (overgrowths). zircons continued to lose lead by diffusion. This is a reasonable explanation for six of the seven points, but CL presents a problem. A 4000 m.y. diffusion curve would be required to encompass this point. Since the GL suite contains zircon with overgrowths, some of the material is probably young (~2700 m.y.) so that the initial material must be >4000 m.y. old. Furthermore, even assuming an age >4000 m.y., the continuous diffusion mechanism would not be consistent with the fact that the sample with, by far, the highest D/a2 (GL) would be the sample least affected by the 2700 m.y. event. If the isotopic fractionation hypothesis were assumed a chord connecting 400-2700 could be drawn for the upper curve. All points (Fig. 16) would fall beneath this line. If the lower line were taken as passing through Nos. GL, 5 and 6 the chord would intersect concordia at 400 and 3200. If the episodic lead loss hypothesis is adopted (Fig. 16) and the kime of lead loss is taken as 120 m.y. (presumably around the time of intrusion of the Cretaceous porphyries), Nos. 27 and GL fall on this line. If another line is drawn between

118

1% J. CATANMRO

and .T.

L. KULP

120 m.y. and samples 5 and 6, the upper intercept is 3120 m.y. which under this hypothesis represents the minimum age of either the time of original formation of these zircons or the age that they were last reset by metamo~hism. During the 2700 m.y. event these older zircons lost varying amounts of radiogenic lead a& young zircon material (overgrowths) were formed. All of the samples then lost some radiogenic lead -120 m.y. age, presumably during the Laramide revolution. Since the 3120 m.y. age is certainly a minimum, it is impossible, in general, to calculate the amounts of lead loss suffered by the in~vidual samples during the 2700 m.y. event. However, two samples, GL and 7, obviously suffered the most severe lead loss (presumably total loss) and the uniqueness of these two sampies

can be accounted for by the episodic lead loss hypothesis. GL has undoubtedly suffered the greatest amount of lead loss ahd, by analogy, was probably most susceptible to the 2700 m.y. event. Sample 7 is the only sample taken from an intrusive (anatectic) granite (R. BUTLER,personal communication, 1959) and was obviously in an ultrametamorphic environment during the 2700 m.y. event. Unfortunately, the data in this case is inadequate to disfinguish among the various hypotheses of the mechanism. Reference to Fig. 7 and Table 3 however, show that there is no relation between the proximity to the Laramide intrusives and the extent of lead loss suffered by the zircons, so that if the episodic lead loss hypothesis is used it must againmsurne a process of lead removal that took place at possibly the approximate time of the Laramide intrusive6 but unrelated to the local temperature gradients. The time of formation of the zircons initially must have been quite ancient. The minimum estimate by any of the mechanisms is about 3100.

Discordant

zircons from the Litt.le Belt., Beartooth

and Santa Catalina Mountains

119

The sequence of events depicted below is taken from ECRELMANN and POLDERVAART (1957). Where possible, probable ages have been inserted. 1. Deposition of Archaean sediments (source rocks >3100 m.y. old). 2. Metamorphism (>3100 m.y.). 3. Intrusion of ultramafic and Stillwater complex. 4. Last metamorphism and pegmatite formation (2700 m.y.). 5. Uplift and peneplanation. 6. Deposition of Paleozoic and younger sediments. 7. Uplift, thrusting and emplacement of felsic porphyry dikes (Cretaceous). Catalina Mountain

Area

The results of age determinations of zircon and muscovite from the Catalina gneiss are summarized in Tables 3 and 4. The zircon results are extremely discordant but suggest an original age >1390 m.y.

Fig. 17. Catalinas Mountain, Arizona samples--continuous plus episodic lead loss interpretation.

diffusion

Figure 17 shows the zircon results plotted on a concordia diagram. Also shown are results previously obtained by SILVER and DEUTSCH (1961) on zircon from the Precambrian-Johnny Lyon granodiorite which is located approximately 60 miles east of the Santa Catalina Mountains. The four points represent analyses of different layers successively, stripped off zircons from one suite, by partial fusions. Regional geologic considerations suggest that the zircons in both localities are probably of the same age. A diffusion curve is passed through the Johnny Lyon samples and gives an upper intercept of 1800 m.y. Note that this curve does not pass through the sample from the Catalina gneiss. Further, it is impossible to put a reasonably aged diffusion curve through this point. If a straight line is passed through the Johnny Lyon and Catalina gneiss samples, the upper and lower

E. J. C:ATANZARO and J. L. KULP

120

intercepts are about 1650 f 20 and 50 I: 20 m.y. The muscovite data indicates an actual metamorphic event in the 30-40 m.y. interval so that episodic lead loss might be expected. Geologic studies in the Santa Catalina Mountains (DUBOIS, 1959 a,b) indicated that the Catalina gneiss had experienced two events. It was considered that the rocks lvere initially formed during the Precambrian and were subsequently remetamorphosed during post-Cretaceous time. The present age results are in agreement ‘The original Precambrian age is reflected by t,he possiblt: with this hypothesis. 1650 m.y. ago of the zircons and the post-Cretaceous metamorphism is indicated by the 32 m.y. K -Ar age of the muscovitc,. Me&nism

of lead loss

In addition to the regularity of the results shown by concordia plots of discordant zircons, one other relationship (or lack of relationship) is well enough substantiated to require explnnation by any proposed mechanism of lead loss: this is the complete lack of correlation between lead loss and “strength” of an event. Three general situations have been found: (1) In a study of the ages of minerals from the Baltimore gneiss, TILTON el al. (1958) showed that an event which caused complete expulsion of argon from biotite did not cause any lead loss from zircon in the same rock. This was also observed in this study in the north end of the Little Belt Mountain area. (2j In the Little Belt Mountains a metamorphic event caused complete argon loss from biotite and hornblende, and complete, partial and negligible (1) lead loss from various samples of zircons (this study). (3) In the Little Belt and Beartooth Mountains (this study) and other portions of the Rocky Mountains (Pikes Peak, ALDRICH et al., 1958) zircons have lost lead (possibly because of a non-metamorphic event) while no significant argon loss occurred in the biotites from the same rocks. The fact that metamorphic events may or may not cause lead loss from zircons is probably due to two variable parameters: the intensity of the metamorphism, including the manner in which it is accomplished, i.e. the presence and composition of solutions, and the degree of metamictization of the zircons at the time of the event,. When lead is lost during a high temperature metamorphic event, the loss can reasonably be attributed to annealing or partial recrystallization with preferential expulsion of the ill-fitting lead atoms. However, this explanation does not seem plausible for the cases in which the lead is apparently lost during a low-temperature, non-metamorphic event and another mechanism must be postulated. It has been established that alkaline solutions can corrode and dissolve zircon (CARROLL, 1953; MAURICE, 1949; STBOCK, 1941). It has also been established that zircons frequently contain significant amounts of water (FRONDEL, 1953; MUMPTOX and ROY, 196 1) and it has been suggested that the water is present as true molecular water and enters the zircon crystal only after a certain amount of metamictization has occurred (MUMPTOX and ROY, 1961). In addition, in a study of a zircon sample* whose isotopic ages suggested extreme .---* Tory Hill, Ontario: Um-Pbao7 age = 1030 m.y.; b2* age -= 1090 m.y.;

Th a3a-Pb20e age -7: 390 m.y.

preferential

PbBm loss, TILTON

U~~J_P~~~’ ago = 1050 m.~.;

Pb*07/

Discordant

zircons from the Little Belt, Beartooth

and Santa Catalina Mountains

121

(1956) showed that the acid soluble lead in the zircon was considerably enriched in Pbzo8 with respect to the total lead in the zircon. Thus, it would seem possible that aqueous solutions might preferentially leach metamict portions of zircons and thereby preferentially remove radiogenic lead. It is not suggested that these solutions need necessarily be hydrothermal in the usual geologic sense of the word. Rather it is suggested that they might consist of alkaline ground water with an increased circulation rate resulting from earth movements accompanying an event. Under these conditions it is possible that the amorphous silica gel of metamictized zircons could be attacked (MUMPTON and ROY, 1961) and radiogenic lead removed from the zircons. On the basis of the results reported here the necessary conditions for low-temperature loss of lead from zircons would appear to include a certain degree of metamictization and some event which chemically and/or physically activat’es ordinary ground water and enables it to leach lead from the metamict zircons. In summary, the necessary and sufficient conditions for the natural removal of lead from zircon are not yet clearly defined but a number of principles have emerged. 1. Some zircons are concordant, hence if the zircon is perfect enough, has low enough effective surface area, and has not been subjected to effective prothe radiogenic lead is quantitatively cesses of leaching or recrystallization, retained. 2. High grade metamorphism may completely remove accumulated radiogenic lead but there are certain metamorphic conditions that do not appear to be highly effective in this process. 3. Selective removal of Pb206 either through radon leakage or the loss of intermediate members of the U238-Pb206 series does not appear to be significant. 4. The degree of discordance (radiogenic lead removal) for a given area and suite of zircons appears to be related to the degree of radiation damage of the crystals. 5. A low-temperature process exists (
events

1. Little Belt Mountains. Three events have been dated in this area: (a) 22470 m.y (an igneous and/or metamorphic event). (b) 1920 m.y. (metamorphism). (c) 120 m.y. (intrusion of felsic porphyries).

E. J.

122

C:ATANZAHO

and J. I,. Kur,r

2. Beartooth Mountains. Two datable Precambrian events are reflected in these rocks: (a) >3120 m.y.* (b) 2700 m.y. (metamorphism and pegmatite formation).

3. Catalina Gneiuu. Two datable events are reflected in this rock: (a) 1640 m.y. (an igneous and/or metamorphic event). (b) 32 m.y. (metamorphism). General The following list of statements completes the summary of the most significant findings of this study: 1. Since different minerals have different susceptibilities to geochemical events, the isotopic analysis of a number of minerals is necessary if complex metamorphic chronology is to be resolved. 2. Age determinations of suites of zircons from a restricted area can yield definitive data for geologic interpretation. With sufficient samples, the time of zircon formation or resetting of the U-Pb chronometers can be uniquely defined. 3. There exists a low temperature chemical process whioh is capable of removing radiogenic lead from zircons. 4. The discordant zircon results obtained in three areas investigated here do not fit the continuous diffusion hypothesis as well as one requiring lead removal at low temperature by ground water. At high temperature episodic lead loss is established. 5. The extent of lead loss can be correlated with uranium content. 6. There appears to be no oorrelation between lead loss and proximity of the samples to Laramide porphyry dikes. 7. Metamorphism of rocks containing previously formed zircons can cause substantial or complete loss of radiogenic lead. 8. Although etook-size igneous intrusives can cause extensive argon loss in biotite (HART, 1961), the analogous effeot of small dikes can be insignificant. 9. Until more studies are made of multiple suites of discordant zircons from restricted areas, and until more is known about the physical and chemioal properties of zircon and the effects of metamictization, the exact meohanism(s) of lead loss remains conjectural. Ac&ncwkdgemente---F. D. ECKEWN end A. POLDERVWT contributed velueble geologic discussionsand G. R. TILT~N and L. T. SILVER contributed signi5c8nt discussionson chemicel techniques end the evaluetion of rcculte. J. R. Brrrzsn, P. E. DABSON, J. C. COBB,F. D. ECKELMANN,B. J. GII.JWIY,A. POLDERVAAXT and J. STEVENS,JR. contributed velueble essietance in the selection end collection of samplca. D. S. MILLER, I. G. SWAINBANX and J. M. WAHPLER helped nkntein the mass spectrometer. A. KAUFDX~, H. MCFADDEN end D. SLAN-Eeseisted, 8t various times, in the zircon separation procedures. R. Ko~oonrvov rn8de the potemium-argon determinetions. The aesisfsnce of M. A. GWINNISB,instrumentmaker, end G. T. PAREER,electronicstechnicicm,is 8ko 8pprWi8ted. J. G. BROKAWd&ted the illustrations 8nd Miss JOAN SONDEBIWM typed the manuscript. Finencial support for this investigetion was provided by the U.S. Atomic Energy Commission under Contmct AT(30-I)-1114. l

Unpublished biotite d8t8.

Discordant, zircons from the Little Belt, Beartooth

and Santa Catalina Mountains

123

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