Garnet peridotite xenoliths in a Montana, U.S.A., kimberlite

19

GARNET PERIDOTITE XENOLITHS IN A MONTANA, U.S.A., KIMBERLITE By B. CARTER HEARN JR.* and F. R. BOYDt

ABSTRACT Wlthm a swarm oflate middle Eocene subsilIclc-alkalic diatremes, one diatreme 270 by 370 m and an associated dike contain common xenoliths of granulite and rare xenoliths of spinel peridotite and garnet peridotIte. Six garnet lherzolIte xenoliths have been found and these show a range of textures. Four are granular, and two are mtensely sheared. Phlogopite IS absent from the intensely sheared xenoliths and is thought to be primary in part m the granular xenolIths. Estimated temperatures and depths of equilibration of xenolith pyroxenes range from 920'~C, 106 km (32 kbar) to l315°C, 148 km (47 kbar). The xenoliths show increasmg amounts of deformation with greater mferred depths of origm. The temperature-depth points suggest a segment of an Eocene geotherm for Montana which is similar m slope to the steep portion of the pyroxene-determined Lesotho geotherm (BOYD and NIXON, this volume) and IS considerably steeper than typical calculated shield and contmental geotherms at present. The steep trend could be a result of plate-tectonic sheanng and magma ascension withm an Eocene low-velocity zone. Preservation of mtensely sheared textures requires rapid transport of material from about ISO km depth during active deformatIOn of relatively dry rock. The occurrence of monticellIte peridotite in this kimberlite diatreme suggests that magmas which crystallized to monticellite peridotIte at relatively shallow depth could be one of the prImitive types of kimberlite magma.

In Montana, U.S.A., one diatreme (the W diatreme) and a nearby dike within a swarm of subsilicic-alkalic diatremes (HEARN, 1968) contain common xenoliths of granulite and rare xenoliths of spinel peridotite and garnet peridotite. The diatreme is 270 by 370 m in plan view, with a rounded irregular shape partially controlled by pre-existing faults. The margin of the diatreme contains a small irregular intrusion of altered monticellite peridotite and contains slices of higher wall rocks of early Eocene age, similar to the occurrences of down-dropped slices in other Montana diatremes which do not contain deep-seated xenoliths. A nearby xenolith-bearing subsidiary dike is in part massive with apparent igneous texture and in part fragmental with pelletal texture. Micas in the dike give K/Ar ages much greater than the maximum intrusive age, as a result of excess argon or xenocrystic origin of micas, but K/Ar dating shows a late-middle Eocene age for several other diatremes and dikes in the swarm. Such an age indicates that the diatreme was emplaced toward the end of widespread subsilicic-alkalic to silicic-alkalic igneous activity in north-central Montana. Although many North American kimberlites contain characteristic xenocrysts probably derived from garnet peridotite, xenoliths of garnet peridotite are extremely rare and have been reported from only two diatremes in Utah (GAvAsCI and HELMSTAEDT, 1969; H. W. WILSHIRE, personal communication), one in Colorado (MCCALLUM and EGGLER, 1971), and one in Quebec (MARCHAND, 1970). Six relatively unaltered garnet peridotite xenoliths from Montana range from about 2.5 cm diameter to 23 x 18 x 12 cm. All six contain * us. Geological Survey. Washmgton, D.C. 20244. U.S.A. t GeophYillcal Laboratory, Carnegie InstItutIOn of Washington, Washington, D.C 247

20008, U.S.A.

248

B.

C.

HEARN AND

F.

R. BOYD

garnet lherzolite mineral assemblages, although diopside forms less than 0.5 vol.% of two xenoliths. The garnet peridotites show distinctive textures: four are granular (either unsheared or showing mortar (necklace) texture of thin zones of fine-grained olivine surrounding large strained olivines; textures similar to the "tabular olivine and enstatite" and "coarse-grained" textures of BOULLIER and NICOLAS (1973)) and two are intensely sheared (Fig. 1) (large rounded to augen-shaped crystals of garnet, strained olivine, orthopyroxene and clinopyroxene in a fine-grained groundmass of granulated olivine and orthopyroxene, with rare small euhedral olivines; a texture intermediate between the "porphyroclastic" and "mosaic" textures of BOULLIER and NICOLAS (1973)). Phlogopite is completely absent in the sheared lherzolites but is present in a variety of textural associations in the granular lherzolites. Possible primary phlogopite occurs as isolated grains; phlogopite also occurs as veins and adjacent to the outer margins of the kelyphite rims on garnets, where it may have originated by recrystallization or introduction in a late-stage process. The occurrence of phlogopite in the granular lherzolites and its absence in the sheared lherzolites are similar to the relations found by BoYD and NIXON (this volume, p. 450) for the lherzolite xenoliths from the kimberlites of northern Lesotho. Phlogopite occurs with diopsides having equilibration temperatures up to but not above about 1200°C in both areas. Nevertheless it is difficult to be sure that this temperature is a meaningful limit because of the difficulty of distinguishing indigenous phlogopite from that introduced at a late stage.

FIG I. (A) PhotomIcrographs of sheared garnet pendotlte xenohths. g, garnet. 0, ohvllle; e, enstatIte; d, dlOpslde. (A) Sheared garnet lherzolite, ~stimated temperature and depth of pyroxene equIlibration 1245°C, 143 km; partIally kelyphitized garnets, large strained enstalltes, diopsIdes and ohvines lie In a matrix of fine-grallled granulated ohvIne and enstatite; matnx texture accentuated by thin dark serpentine coating on olivine grain boundaries; note bent cleavage III amoebOid enstatite in left center; plane-polanzed hght, field of view 2 em across.

GARNET PERIDOTITE XENOLITHS

249

FIG. I. (8) Sheared garnct harzburglte, estImated temperaturc and depth of pyroxene equIlIbratIon 1315 C. 148 km, central large stramed. partIally dlsaggregated olivine crystal lIes m a fine-gramed matn, of granulated olIvme. garnet and ,tramed enstatite ncar border of xelllliith: dark band at upper nght contams granulated garnet, analyzed diopsldc IS tmy mcluslOn (not vI;,lble herc) m en,tatl1e; planc-polarIzed lIght, field of vIew 2 cm acros;,

Electron microprobe analyses (Tables 1 and 2) were made using analytical procedures and data reduction as given by BOYD and NIXON (1972). In the garnet peridotites, olivine and orthopyroxene show a restricted compositional range, F0 9o .s to F0 94 , En 90 .S to En 94 . Clinopyroxenes range from W047Enso.sFsz.s (granular xenolith) to W03z.sEns9.sFss (sheared xenolith). CaMgFe compositions of garnets from three granular xenoliths and from two sheared xenoliths plot within the same fields as Lesotho garnets from sheared and granular lherzolites respectively (Fig. 2). The Cr Z03 content of xenolith garnets ranges from 0.7 to 7.8~/~ and may reflect varying amounts ofCr z0 3 in their source areas or varying modal amounts of garnet and diopside. The compositions of coexisting garnet, diopside and enstatite are plotted in Fig. 3. Five of six analyzed garnet megacrysts, up to 4 cm diameter, have ratios of Mg to Fe similar to xenolith garnets, but with a more restricted range of Ca (Fig. 2); their Cr Z 0 3 contents range from 0.7 to 2.1. Two clinopyroxene megacrysts are 2 cm and 1 cm in size, W047En49Fs4 and W046En49Fss, with 0.35 and 0.42/~ Cr Z 0 3 , much more calcic than diopside megacrysts from Lesotho. An ilmenite megacryst is typically magnesian with composition Il 4s Gk 41 He l I' Temperatures and depths of equilibration of the six garnet lherzolite assemblages were estimated from pyroxene compositions as discussed by BoYD (1973) and were corrected for FeO by the method of WOOD and BANNO (1974), which results in a better trend than is obtained by estimating equilibration pressures from the raw Al z0 3 contents of the ensta-

J-

8028

1955 0 71 46 45 3 943 876 93 n.d.

2997 0 1630 320 441 31 2118 491 0 n.d

Sl Tl Al Cr Fe 2 + Mn Mg Ca Na Nt

Totals-

opx

101.3

57.8 < 0.03 0.79 0.30 4.41 0.08 37.5 0.28 004 010

-

40.9 0.48 20.3 3.03 9.62 0.39 19.6 486 0.07 n.d. 99.3

101.0

gt

40.8 < 0.03 004 < 0.05 7.25 011 52.4 < 0.03 n.d. 0.38

------

01

4032 -

7

3 - - - - --

3016

------- -

- - -----

4027

---

I

0 146 2 1879 0 n.d

981 0

1955 0 32 8 125 2 1889 10 3 -

I -

-

8044

n.d. ----

2975 26 1745 174 585 24 2126 379 10

--

1017

56.6 0.14 1.74 052 6.19 0.13 35.7 0.52 0.18 < 0.1

4046

4035

1927 4 70 14 176 4 1809 19 12 0

12,000; cpx. opx. n

1931 11 165 52 102 3 888 700 194 n.d

=

-

100.9

53.7 040 3.88 182 3.39 011 16.6 18.2 2.78 n.d.

----

opx

H67-28K-4

cpx

971 1 0 0 235 3 1808 I n.d. 8

gt

~--

4000

8032

3011 10 1439 454 382 19 2175 542 0 n.d

=

99.2

41.1 0.18 16.7 7.83 6.23 0.30 19.9 6.91 < 0.03 n.d.

6000; 01. n

3027

=

i

10:': -}

399 0.04 < 0.03 < 0.05 11.6 0.13 49.8 0.03 n.d. O

01

-

opx

---~

-~

01

---- -- --

4034

1947 1 60 26 87 4 1I 12 747 50 n.d.

99.2

4024

1943 2 56 17 141 3 1799 46 13 4

101.3

57.1 0.07 139 0.64 495 0.11 35.4 1.27 019 0.16

3018

3 1850 2 n.d. 8

172

979 0 2 2

995

39.9 < 0.03 0.07 0.11 8.35 0.13 50.5 0.09 n.d. 0.39

- - - ____ -_0- ________

53.5 0.05 1.40 0.91 2.85 0.12 20.5 19.2 0.71 n.d.

H67 28K-I

-----------

cpx

COMPOSITIONS OF COEXISTING MINERALS IN GARNET PERIDOTITES FROM MONTANA -------- - -

Number of catIOns for n oxygens. gt. n

9 9.8

998

AI 2 0

3

Total

5 37 < 0.03 1.66 1.59 149 0.09 7.4 2 2.5 1.32 1.d.

Cr,03 FeO' MnO MgO CaO Na 2 0 NIO

SiO, TiO,

1.

H68-16B

cpx

41.4 < 0.03 19.1 5.58 7.27 0.50 19.6 632 < 0.03 n.d.

gt

TABLE

tv

Vl

o o-<

o:l

::0

:-'1

o

;.Z

Z

::0

~

:r:

n

o:l

o

41.3 0.17 17.6 6.60 6.22 0.30 20.6 6.57 < 0.03 n.d.

994

3003 9 1509 380 379 18 2232 512 0 n.d

8042

Si0 2 Ti0 2 AI 2 0 3 Cr203 FeO" MnO MgO CaO Na 20 NIO

Totals

SI Ti Al Cr Fe 2 + Mn Mg Ca Na Nl

Totals

1003

56.3 0.05 1.41 0.58 4.96 0.11 352 1.36 0.16 0.13 99.9

40.2 < 0.03 0.06 0.09 8.25 0.13 50.7 0.09 n.d. 0.37 99.7

41.5 0.42 18.2 584 6.23 0.29 21.2 5.96 0.03 n.d.

------

4030

4030

1938 I 57 16 143 3 1807 50 II 4 3016

983 0 2 2 169 3 1848 2 n.d. 7

1009

56.6 0.08 1.43 0.61 4.88 0.12 35.5 1.33 0.22 0.15

4025

4033

--------

1935 2 58 16 140 3 1811 49 15 4

12.000; cpx, opx, n

1950 4 88 40 93 4 1088 674 84 n.d.

=

999

54.1 0.13 207 1.39 310 014 20.3 17.5 1.20 n.d.

H69--15F

" Total Fe as FeO.

8041

2997 23 1551 333 376 18 2278 461 4 n.d

Number of catIOns for n oxygens: gt, n

1950 3 70 29 91 4 1103 718 62 n.d.

99.9

54.0 0.10 1.64 1.02 3.02 0.13 20.5 18.6 0.88 n.d.

H67-281-2

I

976 0 1

6000; 01, n

3021

165 2 1867 2 n.d. 7

=

100.1

40.0 < 0.03 0.05 0.05 8.07 0.12 51.4 0.07 n.d. 0.37

4000

8022

3010 37 1827 39 503 18 2248 334 6 n.d.

=

99.9

42.3 0.70 21.8 0.69 8.45 0.30 21.2 4.38 0.04 n.d.

3993

1985 8 135 9 142 4 1032 558 120 n.d.

101.0

56.1 0.30 3.25 032 4.81 0.15 19.6 14.7 1.75 n.d.

4035

1932 4 73 10 184 4 1750 56 22 n.d.

101.4

56.4 0.16 1.81 034 6.40 014 34.3 1.54 0.33 nd.

H67-28K-3

3023

973 0 2 1 223 3 1811 2 n.d. 8

101.7

40.0 < 0.03 0.07 0.07 11.0 0.13 49.9 0.09 n.d. 0.40

Cl

tv

VI

=: en

=l

t""

0

Z

tIl

;><

=l tIl

~

6

:;::l

tIl

'tI

...,tIl

Z

:;::l

;I>

252

B. C. HEARN AND F. R. BOYD

Co 50

GARNETS o Sheared lherZOlite)

60

o

Granular lherzolite

•

Xenocrysts

Montono

70

Lesotho therzolltes granular sheared ,;Q""...

0"-:;"/---

(Boyd a Nixon,

90

'

diamond (Meyer a Boyd, 1972)

\

0

///

'D~--------------)

1973) Inclusions In

"';tr"

( - _ _ _ _ _ ........

'_J

C

-

--

/'

~ xenocrysts, KImberley (Retd a Honor, 1970)

Mg L------;1~0--------=2""0:-------:3~0:----4~0'=-----3!5'=0-F-;>e

FIG. 2. CaMgFe compositions of garnet xenocrysts and garnets from Montana lherzolites, compared to fields of garnets from sheared and granular Lesotho Iherzolites (BOYD and NIXON, this volume), garnet inclusions in diamond (MEYER and BOYD, 1972), and garnet xenocrysts from the Kimberley area (REID and HANOR, 1970).

Co 50 Co- el.I sflnQ 00 met_dlopslde_ensfotlte, Montana Iherzolltes

o

, I

Granular

Mg '--_----U'---'=-

----"'--

-"'----

---"'-

_____="::;---~

50 Fe

FIG. 3. CaMgFe compositions of coexisting garnet, diopside and enstatite assemblages from sheared and granular Montana Iherzolites.

GARNET PERIDOTITE XENOLITHS

253

TABLE 2. COMPOSITIONS OF MEGACRYSTS AND XENOCRYSTS FROM MONTANA ---------

SIze (em)

gt H67 2XJ4 25

gt H67 28B 35

H67-28E 2

H68-17D 0.9

CPX

il H68-16D 07

CPX

--------

SI0 2 TI0 2 AI 2 O, Cr1O, Fe1O, FeO MnO MgO CaO Na 2 0 NIO

42.4 045 215 2.24

41.9 0.49 2l.R 0.74

54.5 0.09 0.43 0.35

54.0 0.07 0.85 0.42

7.00t 0.30 220 4.93 < 003 n.d

8.57t 0.32 20.4 4.52 < 0.03 n.d

2.69t 0.13 17.4 23.6 0.52 n.d.

3.25t 0.16 17.1 22.5 0.89 n.d.

Totals

100.X

99.7

992

98.1

2 874 14 16 221' 468 5 398 0 n.d. 2

987

0.10 48.9 0.51 088 124' 23.6 0.27 11.3 < 003 n.d. 013

Number of catIOn, for n oxygens: gt, n = 12,000: cpx, n = 6000, II. n = 3000 SI TI Al Cr FeJ+ Fe"" Mn Mg Ca Na NI

29X6 24 1783 125

3019 27 1848 42

1989 2 18 10

1981 2 37 12

412 IX 2315 372 0 n.d

516 20 2188 349 0 n.d.

82 4 949 921 37 nd.

100 5 938 887 63 n.d.

Totab

8035

8009

4012

4025

2000t -- -

---

----

, Calculated from the mmeral formula. t Total Fe a, FeO. t Cation total normalIzed mthe course of Fe J + calculation tites. Temperatures and depths range from 920°C, 106 km (32 kbar) to 1315°C, 148 km (47 kbar) (Fig. 4). The xenoliths show evidence of stronger deformation with greater inferred depths of origin. Temperature-depth points for the Montana xenoliths suggest a segment ofa geotherm which is similar in slope to the steep, inflected portion of the Lesotho geotherm (BoYD and NIXON, this volume). Nevertheless, the pronounced change in slope between the sheared and granular limbs of the Lesotho geotherm is not reflected by the six points thus far obtained for the granular and sheared Montana Iherzolites. The Montana trend is considerably steeper than the calculated normal shield and continental geotherms of CLARK and RINGWOOD (1964) (Fig. 5). Sparse heat-flow data east of the Rocky Mountains (COMBS and SIMMONS, 1973; BLACKWELL, 1969) show values of 1.7 hfu in central Montana, 1.0 in northern Montana, and 2.0 in western North Dakota, and thus do not appear to reflect an abnormally steep gradient in the upper part of the lithosphere at the present time. The xenolith trend is also steeper than the calculated geotherm of RoY, BLACKWELL and DECKER (1972) for the Montana plains at 47°N latitude with surface heat flow 1.2 hfu.

254

B. C. HEARN AND F. R. BOYD 1600r----r----r----,-----,-----,---------, /

MONTANA

1400

/

·fG'EOTHERM

~NORTHERN

a->

1200

o

o•

LESOTHO

/

(]) ~

o

::::l -+-

:: 1000 (])

0-

E

(])

r--

800

• SHEARED

o GRANULAR 600

o

50

100

150

200

250

Depth, kilometers FIG 4. Estimated temperatures and depths of equilibration of six garnet perIdotite xenohths from Montana, with the pyroxene-determined geotherm for northern Lesotho (BOYD and NIXON, this volume) for comparIson.

Montono geotherms

1400

u

o

W

II: ::::l

t-

~ 100

w

a.

:E

w

t-

Montana Iherzolltes

600

100

•

Sheared

o

Granular

200 DEPTH,

300

KM

FIG. 5. Estimated temperatures and depths of equilibratIOn of six garnet perIdotIte xenolIths from Montana, and theIr Eocene trend (dotted lme) wIth calculated present geotherms for companson: A, Band C' CLARK and RINGWOOD. (1964), A, shield 1.0 hfu, B, contll1ental, 1.2 hfu; C. contmental. 1.5 hfu. D and E: COMBS and SIMMONS (1973), contmcntal, 14 hfu. D, heat sourcc distrIbution COll"tant: E, heat source distrIbutIOn exponentIal F: Roy et al. (1972), Montana great plall1s, 1.2 hfu

GARNET PERIDOTITE XENOLITHS

255

The xenolith trend is similar in slope to two geotherms calculated by COMBS and SIMMONS (1973), for surface heat flow 1.4 hfu and constant and exponential distribution of heat sources, but is displaced 30 to 50 km to greater depth. The steep segment of a possible geotherm defined by the xenoliths, coupled with deformation textures, could indicate their origin from within and immediately above the low-velocity zone in Eocene time. The increase in temperature gradient compared to normal continental and shield geotherms could be due to the interrelated processes of continental drift, stress heating, and magma ascension. The lack of a kink in the trend between sheared and granular xenoliths may indicate that the base of the lithosphere had begun to heat up by Eocene time. Preservation of textures of intense shearing requires active deformation of relatively dry rock immediately before rapid transport of xenoliths to the surface. The xenolith data suggest that this diatreme erupted material from the mantle down to a depth of the order of 150 km. The association of monticellite peridotite and kimberlite in this diatreme suggests that magmas which crystallized to monticellite peridotite at relatively shallow depths (KUSHIRO and YODER, 1964) may be a primitive type of kimberlite magma-potassic, gas-rich, and derived from mantle depths. Relations between kimberlite and monticellite peridotite elsewhere in the world may have been obscured by secondary processes. The close relationship of kimberlite, carbonatite, and the monticellite peridotitealn6ite suite may define the spectrum of fluids responsible for transport of heterogeneous materials from upper mantle depths to near-surface levels where the resultant mixtures are emplaced as kimberlite.

REFERENCES BLACKWELL. D D. (1969) Heat-flow determinations in the northwestern United States 1. Geophys Res. 74. 992 1007 BOULLIER. A M. and NICOLAS. A. (1973) Texture and fabrIC of perIdotIte nodules from kimberlIte at Mothae. Thaba Putsoa and Kimberley In. Lesotho Kimherlites (editor P. H. NIXON). pp. 57-66. Lesotho National Development Corp. Boyo. F. R (1973) The pyroxene geotherm. Geoc/um. Cosmocllllll Acta (in press) Boyo. F R. and NIXON. P H. (1972) Ultramafic nodules from the Thaba Putsoa kimberlIte pipe Carneyle Inst Wash Yearh 71.362 73 CLARK. S. P and RIN