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Isotopic evidence on the origin of compositional layering in an epizonal magma body
EPSL ELSEVIER
Earth and Planetary Science Letters 136 (1995) 31-41
Isotopic evidence on the origin of compositional layering in an epizonal magma body P.L. Verplanck
a, G.L. Farmer ap*, M. McCurry b, S. Mertzman ‘, L.W. Snee d
a Cooperatiue Institute for Research in Environmental Science (CIRES) and Department ofGeological Sciences, Campus Box 216, University of Colorado, Boulder, CO 80309, USA b Department of Geology, Idaho State University, Pocatello, ID 83209, USA ’ Department of Geosciences, Franklin and Marshall College, Lancaster, PA 17604, USA d US Geological Survey, Denver, CO 80225, USA
Received 24 April 1995; accepted 31 July 1995
Abstract A detailed isotopic study of the Oligocene age (36 Ma), alkaline composition Organ Needle pluton in south-central New Mexico was undertaken to test models for the generation of compositional layering in silicic, epizonal magma bodies. The pluton is isotopically heterogeneous with its alkali feldspar granite composition cap (73-76% SiO,) having lower initial eNd and higher 87Sr/ ‘?jr ratio than the underlying main syenite (-5 vs. - 2 and - 0.709 vs. - 0.706, respectively). Both lithologies have isotopic compositions significantly different from those of the Precambrian granite wall-rock (Ed,, - - 12.1 and “Sr/ “Sr - 0.784 at 36 Ma). The isotope data indicate that none of the lithologies of the pluton represent the products solely of roof or wall-rock melting, and that the capping granite could not have been derived in a closed-system from differentiation of the underlying syenitic magma. However, the capping granite has isotopic compositions similar to those of a chemically heterogeneous inequigranular syenite found along the margin of the pluton at its deepest exposed level (4-6 km paleodepth). Field observations, and new 4oAr/ 39Ar age determinations, confirm that this lithology was comagmatic with the remainder of the pluton. We conclude that the capping granitic magma was derived from buoyant lNd - - 5 magma rising along the margins of the magma chamber and that the inequigranular syenite preserves a remnant of this sidewall magma. The main syenite body was encapsulated in this marginal, lower eNd, magma, while undergoing closed-system differentiation, most likely from a mafic progenitor. The Ed,, = -5 magma may represent mafic magma initially equivalent isotopically to the main syenite but which subsequently assimilated Precambrian wall-rock (- 10% by mass) at the base of the magma system.
1. Introduction The development of compositional zonations and/or layering is now generally accepted as com-
* Corresponding
author.
[email protected].
Fax:
(303)
492-1149;
email:
mon, if not ubiquitous, occurrence in silicic magma bodies [l]. Multiple processes are likely to be involved in the production of compositional heterogeneities in a given magma body, including melting of the magma chamber roof and/or wall rock [2], the upward movement and ponding of differentiated magma produced at the chamber base and/or sidewalls [3], and the sequential input into a shallow
0012-821X/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0012-821X(95)00147-6
32
P.L. Verplanck et al. /Earth and Planetary Science Letters 136 (1995) 31-41
magma chamber, and independent differentiation of, magma derived from greater depth [4]. Determining the relative importance of these processes for a particular magma body is hampered by the fact that only imperfect samples of the body can generally be obtained. Ash-flow tuffs may represent selective and incomplete sampling of the actual epizonal magma body [5] while exposed epizonal plutons suffer from late-stage differentiation or reequilibration [l]. Recently, however, it has been demonstrated that the Tertiary age, epizonal Organ Mountain batholith in south-central New Mexico contains at least one well-exposed pluton, the Organ Needle pluton (ONP), that has preserved both continuous and discontinuous compositional variations virtually identical to those inferred for pre-eruption magma bodies parental to many large volume, silicic ash-flow sheets [6]. As a result, the pluton represents a rare opportunity to study the origin of chemical layering and zonations in silicic magmas. In addition, because the Organ Needle batholith dominantly intrudes Precambrian granitic rocks which, from preliminary studies, are known to have Nd and Sr isotopic compositions markedly different from the Tertiary plutons [7], the ONP is ideally suited for studies of the origin of isotopic zonations in silicic magma bodies. In this study, we use combined elemental and Sr and Nd isotopic data to assess the origin of chemical and isotopic variations in the epizonal magma body from which the ONP crystallized.
2. Geologic setting The Late Oligocene Organ Mountain batholith is exposed in the Organ Mountains of south-central New Mexico (Fig. 1). The Organ Mountains represent a Late Tertiary fault block bounded by high-angle normal faults and tilted westward by 15-20“. Erosion of the fault block has exhumed a three-dimensional view of the epizonal intrusive rocks and the overlying Organ Mountain caldera with attendent silicic volcanic fill. Recent field, petrographic, and geochemical studies suggest that the youngest ashflow tuff, the tuff of Squaw Mountain, was deposited at N 36 Ma [81 and tapped the uppermost, highly siliceous portions of a shallow-level magma body represented by the ONP [6,9].
m
Younger Plutons
m
ONP SiO2 > 73 %
0
ONP SiO2 S&71% ONP SiO2 < 58 %
m
Precambrian Cranib
lb
_I
1kd h
Fig. 1. Geologic map of Organ Needle batholith (after Seager and McCurry [6]) showing spatial variations in whole-rock lNd values. The plus symbols in inset map delineate approximate area1 extent of inequigranular syenite. Qaf = Quatemary alluvium.
The ONP itself intruded near-surface and is now continuously exposed to a paleodepth of 4-6 km. At the structurally highest portions of the pluton, the ONP intrudes Precambrian granitic rocks, Paleozoic sedimentary rocks and, locally, its own volcanic cover, while at greater paleodepths it intrudes Precambrian (1.4-1.6 Ga) granitic rocks of the White Sands pluton ([lo]; Fig. 1). The ONP is alkaline in overall composition but shows marked compositional layering. The bulk of the pluton consists of syenite that is underlain by, and separated by a broad gradational contact from monzonite. The syenite is overlain by, and separated by a narrow compositional gradient from, an alkali feldspar granite (the intrusive equivalent of the Squaw Mountain t&f). The alkali feldspar granite (AFG) shows a narrow range of chemical compositions from 73 to 76% SiO,, but existing [6] and new chemical data (this study) reveal that the syenite is vertically zoned from N 57% SiO, to at least 68% SiO,. The continuous chemical zonation in the equigranular syenite is manifested by a general upward decrease in plagioclase anorthite
P.L. Verplanck et al. /Earth
and Planetary Science Letters 136 (1995) 31-41
content and the appearance of alkali feldspar, both as individual grains and as replacement of plagiclase, in rocks with SiO, > 65% [6]. Clinopyroxene (Ca-rich augite) is present in deeper and more mafic portions of the syenite, while hornblende and biotite occur over a much broader compositional range. Table 1 Isotopic and selected chemical SAMPLE
Si@l
While the chemical layering and zonation in the ONP were known from earlier studies, our new field observations along the easternmost, and structurally deepest, exposed portions of the batholith revealed that significant lateral compositional variations occur in the pluton as well. At this level in the pluton, we
data from the Organ Needle pluton
CaO
WA)
33
Wk)
K20
Na20
R@
(wt%) (wt%) @pm) @;I
87Sr/3 86Sr
B7Srf 4
Sm
Nd
86Sri
@Pm)
@Pm)
143Nd/5 144Nd
m4
ENd
W!lB!z jnea ui wan&r PV4391
57.0
4.77
3.84
5.05
78.5
676
0.70983+1
0.70966tl
16.6
94.6
0.512362+10
-4.97
PV5492 PV6491
58.3 60.5
3.91 3.31
4.75 4.29
5.18 5.75
80.2 65.6
626 1094
0.71006*1 0.70821+1
0.70987+1 0.70812~1
12.9 7.81
70.7 46.1
0.512338+9 0.51245tlO
-5.46 -3.23
PV4992 PV7591
61.4 61.6
2.65 2.43
5.29 5.57
5.56 5.63
98.3 101
3% 402
0.70936~1 0.7092&l
0.70899+1 0.70889+1
12.7 10.9
77.9 64.0
0.512386rlO 0.512367+10
-4.47 -4.86
PV6391
62.7
2.39
5.29
5.62
83.1
739
0.70824*1
0.70808tl
9.57
58.2
0.512394+10
-4.31
PV7291
63.2
1.68
6.27
5.39
107
314
0.71007+1
0.70957tl
22.4
94.2
0.512356+10
-5.26
PV5292 PV5091
63.6 65.6
2.20 2.40
5.54 4.66
5.22 4.67
115 135
347 380
0.70986+1 0.70729=1
0.70937+1 0.70677~1
12.9 9.14
77.7 53.4
0.51238+6 0.51245tlO
-4.59 -3.24
PV5092 :ouimonulm
67.0
1.03
5.90
5.49
144
155
0.7108422
0.7094723
16.6
108
0.512375~9
-4.65 -2.46
PV4392
62.8
2.32
5.23
6.28
63.9
741
0.70691+1
0.70678tl
7.60
46.2
0.51248929
PV6591
62.9
2.35
5.14
6.06
77.4
639
0.7066%1
0.70645tl
6.00
37.8
0.512494=10
-2.35
5309207 5309206
64.6 65.5
2.33 1.63
5.17 5.56
5.01 5.57
133 139
423 331
0.70585t2 0.7062Oil
0.70538i2 0.70557+1
10.2 8.44
59.6 49.4
0.512484+7 0.512473+7
-2.57 -2.79
5279202
66.6
1.63
5.41
5.31
214
291
0.70660*1
0.7055122
9.39
54.6
0.5125Olt7
-2.25
5279201
66.7
1.72
5.68
4.56
231
362
0.70635+1
0.7054Ot2
9.84
57.9
0.512491+7
-2.44
5299202
67.2
0.87
6.07
4.94
254
282
0.70701+2
0.70564~2
10.2
59.1
0.512471+10
-2.83
4lkali feldsorr
eranitp
PV35Yl
73.1
0.29
6.98
2.85
262
83.3
0.71453zl
0.7098Yz7
9.63
55.2
0.51235728
-5.06
PV3691
73.3
0.23
5.90
3.82
202
84.7
0.71447+1
0.71094+5
10.7
61.5
0.512365+8
-4.90
PV37Yl
73.5
0.23
6.72
2.98
250
100
0.71476+10
0.71105rlll
8.16
48.7
0.512354~10
-5.10
PV292
74.4
0.27
6.93
2.78
349
32.3
0.72905~2
0.71310+20
8.59
47.6
0.512358+9
-5.06
391
27.8
0.7360524
0.71537r30
5.00
31.1
0.51235~9
-5.16
PV192 76.1 0.36 5.97 3.16 P m U iscellaneous-mafic mclnw in ine6-iui9mnulnr PV6191 47.9 9.63 4vemae Premm6rian mnitic 68
2.2
1.18 rnck 5.0
suede
4.00
27.1
231
0.7154&l
0.71531
7.41
28.2
0.512525tlO
-2.03
3.3
238
189
0.78631
0.78398
21.7
110
0.511999
-12.1
’ Oxide concentrations by X-ray fluorescence at Franklin and Marshall College. Normalized to 100% volatile-free. ’ Trace element concentrations bij isotope dilution. Uncertainties are - 1% fot Rb, Sr and 0.5% for Sin, Nd. 3 *‘Sr/ s6Sr ratios were analyzed using 4-collector static mode measurements on a Finnigan MAT 261. Twenty-seven measurements of the SRM-987 standard during the study period yielded a mean 87Sr/ s6Sr = 0.71032 f 2 (2~ standard error). Measured s’Sr/ 86Sr ratios were corrected to SRM-987 of 0.71028. Errors are 20 of mean and refer to the last two digits of the 87Sr/ =Sr ratio. 4 Initial isotopic composition calculated at 36 Ma. lNd values calculated using 143Nd/ 144Nd (CHUR) = 0.512638. 2~ errors on Sr initial iostopic compositions represent combined errors on measured Sr isotopic compositions and on Rb and Sr concentrations. ’ 143Nd/ 144Nd ratios were normalized to 146Nd/ 144Nd = 0.7219. Analyses were dynamic mode, three collector measurements. Thirty-one measurements of the La Jolla Nd standard during the study period yielded a mean ‘43Nd/ ‘44Nd = 0.511838 + 8.
P.L. Verplanck et al. /Earth
34
1
and Planetary Science Letters 136 (1995) 31-41 400
n
Alkali feldspar granite
. . . .
300
....
n
-
0
0.0
syenite .
.
.
.
’ * . * . ’
CD,
.
.
.
n
r.g .., ~ ,1,
.
’ . . . .
+
8
200 -
.
. . . .
Rb
.
Equigranular syenite 0 (interior)
. . . .
I
* * . .
.
.
.
.
1
.
.
.
,
I
.
.
.
.
I
.
.
.
.
Sr
6.0
1000 -
S.o&
Na20
750 -
0
i 500
0
0
0
-
rot
l
250 -
I
0
Nd
100
.
80
m nw
60
n 40
0
w L
.
.
.
8
.
I
.
.
.
I
0% 00
.
.
.
.
I
E
.
.
Nd
.
.
, . . . .
20
:
:
I
.
.
.
.
I
.
.
.
.
I
87Sr /86Sr
.
.
.
.
n
0.7150
n
0.7125
I 0.7100
0. .
0.7075
55
65
70
7s
80
.
0
00
0.7050 60
l+ l
yq, .6.0 e
8
55
60
65
,
a0 70
SiO2
75
i 80
P.L. Verplanck et al. /Earth
and Planetary Science Letters 136 (199s) 31-41
identified a border lithology of the ONP that is exposed along a 200-400 m wide band directly adjacent to the wall-rock contact (Fig. 1). This lithology is an inequigranular syenite characterized by subhedral plagioclase phenocrysts partially to completely replaced by perthitic potassium feldspar set in a finer-grained matrix of quartz, potassium feldspar, hornblende, biotite and clinopyroxene and by variable and high abundances of accessory phases, particularly sphene and apatite. The inequigranular syenite contains rounded mafic enclaves, as does much of the pluton, and thermally metamorphosed, possibly restitic, xenoliths of disaggregated Precambrian mafic dikes and granite. Texturally, the transition between inequigranular and the coarse-grained equigranular syenite is gradational.
3. Analytical procedures and results We obtained Nd and Sr isotopic data, and Rb, Sr, Sm, and Nd concentrations (Table 1, Fig. 2) from multiple samples of the equigranular syenites, the capping alkali feldspar granite, and the inequigranular border syenite, to determine the relationships between these lithologies of the ONP. In addition, isotope and selected trace element data were obtained from six samples of the Precambrian wall-rock of the pluton, and from one mafic enclave from the inequigranular syenite. Initial (36 Ma) Nd and Sr isotopic compositions are referred to in the following text. Among the seven samples of the equigranular syenite analyzed, Sr abundances decrease (740-282 ppm), while Rb (64-254 ppm), Sm (6.0-10.2 ppm), and Nd (38-59 ppm) abundances increase with increasing SiO, (Fig. 2). Over the range of SiO, represented by these samples (63-67%) there are essentially no Nd isotopic variations ( lNd = - 2.3 to -2.8; Fig. 1, Fig. 2) and only a restricted range of 87Sr/86Sr (0.7054-0.7068). The two samples with 87Sr/ 86Sr > 0.7060 (PV4392 and PV6591) were obtained closest to the margins of the pluton (Fig. 4). Unlike the equigranular syenite there is no obvi-
Fig. 2. Selected major, trace and isotopic compositions
35
ous correspondence between chemical composition and geographic position in the capping AFG. However, over the range of SiO, sampled (73-76 wt%; Table l), the Sr and Rb concentrations continue to decrease (loo-28 ppm) and increase (262-391 ppm), respectively, relative to the most silicic portions of the underlying syenite. Both the Sm and Nd concentrations of the AFG decrease with increasing SiO, (11-5 ppm and 62-31 ppm). The Sr isotopic compositions of the AFG (s7Sr/ @jSr = 0.7099-0.7153) are significantly higher than the values determined for the equigranular syenite, with the highest 87Sr/ “Sr corresponding to the highest Rb/Sr, lowest Sr content samples (Fig. 4). The lNd values of the AFG are uniform (-4.9 to -5.2) and distinctly lower than the values determined for the equigranular syenite. Despite its limited geographic extent, the inequigranular syenite is grossly heterogeneous chemically (SiO, = 57-67%,) and trends towards more mafic compositions than the immediately adjacent portions of the equigranular syenite. Rb concentrations generally increase and Sr contents decrease (66-144 ppm, and 155-1094 ppm, respectively), with increasing SiO,. In contrast, the LREE abundances are not correlated with SiO, and are higher than those observed in other portions of the pluton (Sm = 7.8-22.4 ppm, Nd = 46-108 ppm). The inequigranular syenite shows a range of Sr and Nd isotopic compositions (87Sr/ 86Sr = 0.7068-0.7099, and cNd = -3.2 to -5.5), but the majority of the samples have Ed,, values I - 4.5 and 7Sr/ 86Sr 2 0.709. The one rounded mafic enclave analyzed from the inequi ranular syenite has low SiO, (47.9%) and 8F high Sr/ 86Sr (0.71548), but Ed,, (- 2.03) distinct from that of the enclosing syenite (Table 1). The surrounding Precambrian granitic rocks have significantly lower ENd values and higher 87Sr/ 86Sr ratios (average eNd - - 12.1, and 87Sr/ 86Sr - 0.784, at 36 Ma; Table 1) than those of any of the lithologies of the ONP. New 4oAr/ 39Ar age determinations were obtained to place age constraints on the inequigranular syenite. Biotite, potassium feldspar, and hornblende sepa-
vs. wt% SiO, in various lithologies
of the Organ Needle pluton.
36
P.L. Verplanck et al. /Earth
and Planetary Science Letters 136 (1995) 31-41
ployed to produce the 4oAr/ 3gAr incremental release spectra (Table 2; [ 111). The 4oAr/ 3gAr age determinations from hornblende, biotite, and potassium feldspar from the inequigranular syenite (sample PV5292) yielded
rates from a sample of the inequigranular syenite (PV5292) were irradiated (Table 2, Fig. 3) for this purpose. A sanidine separate of known age 181 from the Squaw Mountain tuff was analyzed as an intralaboratory comparison. Standard techniques were em-
Table 2 4oAr/ 39Ar analytical
data, Organ Mountain
Batholith,
New Mexico
29.0
RYIP __~
0.8!s4 1.0128
03425 0.4040
2.51 05057 2.49 13954 055917 250 l&lop 2.8705 1.15626 2.483 105LW 35014 1.40320 2.4% llav 2.9888 1.20201 2.4% llsop 35520 1.43fn5 2.483 124W 2.8626 1.14959 2.490 lvop 1.6141 0.64522 2.50 14X@ V529Xymite-lowerborderme-Potassiumfeldspar
9lXlP 55OP
IL%99
Es$F..~t~~~.~~* E !z lE3 1050 1100 1150
1.0203 EE 1:412a 15639 12108 1.9706 27286 4.4766 16.120
0.12926 0.64675 1.0143 :z ;mzE i.1949
3.386 1.578 1.685 1.752 1.8)’ Ez 2:m
22. 23. 23. 24. 25. 24. 27. 25. 73.
59.1 94.0 97.4 973 975 97.4 %.7 955 895 802 69.9
!I
49.8 83.7 74.8
E :;:
2’G 2.487
E 58.
Ez :z 4:1m 2 1350 ll”s 0.65333 5;: 4;. 1450 0.95816 0.38598 2.482 18. r’5292-Sy&1clower to&r zone-Biotile >tal as date: 33.91 t 0.19 Ma; plateaudate:34.78t 0.09Ma O.Cih&+ 0.25%; ;;vj6 mg 0.0350 0.69 19. 0.17649 0.1181 1.49 62. E 0.84472 0.37Bo8 2.234 129. 0.76W9 0.31451 2.427 139. E 0.52115 0.21w 2.48 113. loo0 0.52515 0.2117 2.48 95. 1050 0.90253 0.3620 2.49 15280 0.62flO6 2.433 E: 1lOOP 1.9039 0.77684 2.451 18. 115aP 1.6781 0.68485 2.450 8.2 13oop
1.0 $3 4.4 5.6 6.1 12.7 15.4 132 15.7 12.6 7.1
0.6 :3 2.7 3.8
E 912 87.8
:::
::;
:.“9
Zf fU.8 E3
7.2 24.1 69.2 79.4 79.0 75.0 77.3 79.5 88.0 90.4
i.; 32 1.9
::; 10.2 R.5 5.6 5.7 9.7 lb.9
20.9 18.4
365 t 05 36.0* 0.4 355 t 03 35.7* 02 35.47t 0.17 3553f 0.15 3535* 0.08 3553* 0.09 35.40* 0.07 353s t 0.08 35.4st 0.09 35.6f 0.2
47.48t 0.17 22.28l 0.12 23.78l 0.06 E::
: 8%
z:::: 30:73t 32.16t 33.02t 35.00* 33.65* 3431f 34.94t
0:10 0.06 0.06 0.05 0.0s 0.14 0.16
9.9t 1.1 21.3t 0.3 31.80t 0.0s 34.46t 0.10 35.3t 0.4 35.3* 0.5 3.55* 0.3 34.60t 0.07 34.8st 0.09 34.85t 0.10
.E’Ma:kochron qe: 35.6f 0.3Ma 0.33 0.53 0.70 1% 102.5 1050 1075 1100 112.5 11sOP 1175P
0.0075 00244 om37 0.0401 EG 055405 245% 20036 1.1680 1.4816
0.00&l 0.0136 0.0120 0.0173 0.0248 0.0440 !“9715’: 0.78825 0.45696 057937
E I!
2.523 2520 2.556 2.5%
::: 0.32
0.16 0.16 0.16 0.16 0.15
6.6 6.1 20.2 9.3 22.8 27.2 36.7 45.3 59.1 77.9 89.6 94.2 92.7 92.2
0.2 0.6 0.5 Lx 0.4 ::‘8 1.4 2.; 24.8 18.2 14.4
27.t 8. 33.* 2. 32.: 32 13.r2. 26.: 3. 2.8.*2. 33.* 3. 33.l 2. 34.6s1.4 36.60* 0.10 35.71* 0.10 35.67t 0.08 36.u t 0.25 36.18 r 0.19
P.L. Verplanck et al. /Earth
r-----1
r
200 r
r
10 I1
__--___+__ _I
_--__I
r _r - ’
r'
20
30
40 I
MI1
60
70 1
80
90
I
100
-AK, Released (%)
Fig. 3. 4oAr/39Ar the inequigranular
37
4. Discussion
,=--;1
r
-1
and Planetary Science Letters 136 (1995) 31-41
incremental release patterns for samples syenite and the Squaw Mountain tuff.
from
“‘Ar/ 39Ar plateau ages of 35.69 + 0.09 (1 a> Ma, 34.78 f 0.09 Ma, and no plateau, respectively (Fig. 3). Sanidine from the Squaw Mountain tuff yielded a plateau age of 35.46 + 0.14 Ma, which is virtually identical, within the specified errors, to the 35.75 f 0.12 Ma plateau age from the previously determined for this sample [8]. We interpret the hornblende plateau age as most closely representing the crystallization age of the inequigranular syenite, due to its high closure temperature for the retention of radiogenie Ar [12].
The large Sr and Nd isotopic contrasts between the pluton and the enclosing Precambrian granite (Table 1) preclude the possibility that any of the various intrusive lithologies, including the capping AFG, originated solely by partial or complete melting of Precambrian wall- or roof-rock. Instead, each portion of the pluton investigated, including the silitic cap, must have crystallized from magma injected into the upper crust that was derived from a source isotopically distinct from the felsic Precambrian crust. The ubiquitous presence of rounded mafic enclaves in the ONP, and the field evidence that the equigranular syenite was underlain by monzonitic magma in the pre-eruptive magma chamber, further suggest that mafic composition parental magma(s) were involved in the production of the ONP. A similar conclusion has been reached for many other Cenozoic silicic magma systems in the western United States [ 131. A direct derivation from mafic parental magma seems most likely for the equigranular syenite, given the similarity in the Nd isotopic composition of the equigranular syenite and the one mafic enclave analyzed (i.e., lNd N -2; Table 1). The uniform initial
Notes to Table 2: Mineral separates were prepared by magnetic separator, heavy liquids, and hand picked; grains were 60-120 mesh size (250-125 pm). Seperates were then hand picked to greater than 99% purity, cleaned in reagent-grade acetone and de-ionized water in an ultrasonic bath, air dried, wrapped in aluminum foil boats and sealed in silica vials along with monitor minerals prior to irradiation. Samples were irradiated at the TRIGA reactor at the US Geological Survey, Denver, CO. ’ Abundances of “Radiogenic 4oAr” and “K-derived 39Ar” are reported in V as measured on a Mass Analyzer Products 215 Rare Gas mol Ar per V of signal. Detection limit at the time of the mass spectrometer. Conversion to moles can be made using 9.736 X lo-l3 4oAr” and “K-derived 39Ar” are experiment was 2 X lo-t7 mol Ar measured on a Faraday detector. Analytical data for “Radiogenic caculated to five places: 4oAr/ 39Ark is calculated to three decimal places. “Radiogenic “OAr”, “K-derived 39Ar”, and “40Ar/ 39Ark” are rounded to significant figures using analytical precissions. Apparent ages and associate errors were calculated from unrounded analytical data and then rounded using associated errors. All analyses were done in the Argon Laboratory Geological Survey, Denver, CO. Decay constants are from Stieger and Jager [U]. The irradiation monitor MMhb-1 (1.555 wt% K), with an age of 520.4 Ma [16] was used to calculate J values for the analyses. ’ Argon isotopic composition is corrected for volume, mass fractionation, trap current, radioactive decay of 37Ar and irradiation produced 39Ar and interfering Ar isotopes. Production ratios measured on pure K,SO, and CaF, salts irradiated with the samples were used to correct for irradiation-produced aAr (from K) and 39Ar (from Ca). Corrections for Cl-derived 36Ar were determined using the method of Rodick [17]. Production ratios determined for all samples were: “ArsrJ 39Arx, 1.10 X 10W4; 38Ark/ 39Arx, 1.306 X lo-‘; 4oArk/39K, 7.8 X 10-3; of split gas fractions from each 39Arc,/ 37Arc,, 5.95 X 10m4; 36ArK/37Ca, 2.70 X 10m4; 3 Ark/ 3sArc,, 2.4 X lo-‘. The reproducibility monitor (0.15%, lo) were used to calculate imprecisions in J. J values were interpolated from adjacent monitors and have similar uncertainties to the monitor. Uncertainties in calculations for the date of the individual steps in a spectrum were calculated using equations yf Dahymple et al. [18]. To calculate apparent K/Ca ratios, divide the 39Ar/ 37Ar value by 2. p Fraction included in plateau date. l
P.L. Verplanck et al. /Earth and Planetary Science Letters 136 (1995) 31-41
38 0.716,
0.709lr
0
I
I
I
I
1
10
20
30
40
87Rb/86Sr
(36 Ma)
Fig. 4. *‘Sr/ “9 (36 Ma) vs. s7Rb/s6Sr for various lithologies of the Organ Needle pluton. Symbols as in Fig. 2. Inset diagram shows data for equigranular and inequigranular syenites-circled symbols are samples of the equigranular syenite closest to the pluton margin. Simple mixing and assimilation-fractional crystallization (AFC) modeling described in text. D values are bulk partition coefficients for Sr and Rb, and r is ratio of rate of assimilation to rate of crystallization. Tick marks mark variations in F, the fraction of magma remaining, and A4, /Mi, the ratio of the mass of assimilated crust relative to the original magma mass 1221. The simple mixing curve shown approximates the best-fit line through the AFG data, which corresponds to an apparent isochron age of * 10 Myr.
isotopic compositions of the equigranular syenite, in turn, implies that the range of bulk compositions observed within the interior, syenitic portions of the pluton was produced in a system closed to wall-rock interaction. The mechanism of magma differentiation that produced the range of bulk compositions in the interior portions of the pluton remains unresolved, although the major element variations in the equigranular syenite from 60 to 68% SiO, are mimicked by fractional crystallization of the observed mineral assemblages [6]. The relationships between the AFG and the inequigranular syenite, and either a mafic progenitor or the equigranular syenite are more obscure. An important observation from the AFG, for example, is that, unlike the equigranular syenite, significant variations exist in its initial Sr isotopic compositions. These isotopic variations are correlated with Rb/Sr ratios and yield an apparent isochron “age” of 10 Myr (Fig. 4). It is unlikely, however, that the AFG could represent an older intrusive body into which the magma parental to the equigranular syenite intruded,
as implied by the isochron age, given that the AFG itself intrudes the Cueva Tuff which has been dated at 36.2 Ma [8]. It is equally unlikely that the apparent isochron represents a magma residence age for the AFG, as it is unreasonable that the parental magma could have remained partially molten and in isotopic disequilibrium over 10 Myr even with recharge of basaltic magma to the base of the system [14]. Instead, the apparent isochron is most likely a byproduct of magma-wall rock interaction, a process not recorded in the isotopic compositions of the interior portions of the pluton. The correlation between increasing initial 87Sr/ 86Sr and decreasing Sr contents in the AFG is similar to that observed in many silicic ash-flow tuffs and is typically interpreted as evidence that magma-wall-rock interaction occurred through twocomponent mixing [15,16]. Simple mixing between the average White Sands pluton (Table 1) and a silicic magma with 87Sr/86Sr of 0.710, but which has developed a range of Sr concentrations (e.g., from 100 to 30 ppm) and Rb/Sr ratios, could generate a range of correlated Rb/Sr and initial 87Sr/ “Sr ratios similar to those observed in the AFG after only 1% bulk assimilation (by mass) of the wall-rock material. Concomittant assimilation and fractional crystallization (AFC; [17]) could also account for the correlated Sr isotopic compositions and Rb/Sr in the AFG, at low rates of assimilation relative to crystallization (r < 0.3). In this model, the range of initial isotopic compositions in the AFG represents variable amounts of assimilation, with samples having the highest “Sr/ 86Sr having assimilated the highest amount of crust relative to its initial magma mass (a maximum M,/Mz = 0.01 if the same initial magma and wall-rock compositions used above are assumed and using D,, = 2.5 and D,, = 0.5; Fig. 4). While the exact amounts of assimilated crust calculated in either AFC or simple mixing models obviously depends on the mechanism of assimilation (bulk vs. selective, for example), it remains valid that small amounts of wall-rock assimilation can account for the Sr isotopic variations, and for the uniform lNd values, of the AFG. In any case, removing the isotopic effects of the N 1% wall-rock interaction needed to account for the internal range of Sr isotopic compositions in the AFG still leaves a distinct Sr and Nd isotopic contrast between the AFG and
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and Planetary Science Letters 136 (1995) 31-41
the equigranular syenite. This contrast implies that the AFG is not the end-product of the closed-system differentiation processes that produced the range of chemical compositions observed in the underlying equigranular syenite. Similarly, the Nd and Sr isotopic contrasts between the inequigranular and equigranular syenites preclude a simple closed-system relationship between these two lithologies, despite the gradational contact between the equigranular and inequigranular syenites and the 35.7 Ma ““Ar/39Ar hornblende plateau age for the latter, which imply that the magmas parental to both lithologies were contemporaneous. For example, the inequigranular syenite cannot represent the product solely of sidewall crystallization [3] of magma from which the interior syenite crystallized. It is also unlikely that the border lithology formed by mixing between magma isotopitally equivalent to the interior portions of the syenite and partial melts, restite, xenoliths, or xenocrysts derived from the immediately adjacent Precambrian granite wall-rock. While a range of major and trace element compositions for the inequigranular syenite might be produced by such mixing, a wide range of Sr and Nd isotopic compositions would also be generated. For example, simple mixing of bulk wall-rock and magma equivalent to the equigranular syenite produces a strong covariation between eNd and l/Nd (Fig. 5). A similar covariation would be
I oma
Omo
l/Nd
OMO
o.oM
(ppm)
Fig. 5. end (36 Ma) vs. l/Nd (ppm) for various lithologies of the ONP and for the Precambrian granite wall-rock. Ticked marked line is two-component mixing line between equigranular syenite (e Nd = -2.5) and average. Precambrian granite (boxed numbers are fraction of latter in mixture).
39
Fig. 6. Cartoon depicting model in which silicic cap of the Organ Needle pluton develops through accumulation of differentiated magma initially produced at the base of the magma system but which migrated upwards along the magma chamber walls. Numbers shown are eNd values for wall-rock, inputted mafic magma(s), and various lithologies of the ONP.
expected if the range of Nd contents in the inequigranular syenite were due to the mechanical incorporation of LREE-bearing accessory phases derived from the wall-rock. The observation is that the Nd isotopic composition of the border lithology is relatively uniform over a wide range of Nd concentrations (Fig. 5). The isotopic compositions of the inequigranular syenite (eNd = - 5, and 87Sr/ 86Sr N 0.709), however, are identical to those inferred for the magma parental to the AFG (prior to minor wall-rock interaction). Therefore, while neither lithology could have been derived from the equigranular syenite, the isotopic data allow for a direct genetic relationship between the AFG and inequigranular syenite, despite a spatial separation of at least 3 km in the original magma chamber. Such a relationship can be best accomodated in a model in which the development of the compositional layering observed in the ONP involved the upward migration of intermediate to silicic composition magmas along the margins of the magma chamber (Fig. 6; [18]). In such models, compositionally diverse magmas are produced by fractional crystallization near the base of an epizonal magma body [19] and then rise buoyantly along the
40
P.L. Verplanck et al. /Earth
and Planetary Science Letters 136 (1995) 31-41
chamber sidewalls, the most silicic magmas ( > 70% SiO,) reaching the top of the magma system [18]. We consider the inequigranular syenite to represent a remnant of the differentiated magmas that rose along the chamber sidewalls, and the AFG to represent the accumulation of silicic magma that traveled to the top of the chamber by this same route, athough we recognize that the exact disposition of various magma compositional types in the original magma chamber may have been modified during the eruption of the Squaw Mountain tuff. This model is attractive in that it accounts for the similar isotopic compositions of the AFG and inequigranular syenite and: (1) accounts for the wide range of bulk compositions observed in the inequigranular syenite over the restricted area over which it is exposed (- 1 km2), and the overall lack of field evidence for significant sidewall crystallization along the margins of the pluton. (2) is consistent with preliminary data from shallower structural positions in the pluton that reveal that lNd = -5 syenite is present, is restricted to the pluton margins, and increases in average silica content with decreasing depth [20]. (3) allows for the wall-rock interaction recorded in the AFG to have occurred during transit along the margins of the magma body, a process consistent with the occurrence of partially disaggregated Precambrian granite, in various stages of local bulk assimilation, along the margins of the ONP. The equigranular syenite is envisioned as chemically evolvin separately while encapsulated in lower A lNd, higher Sr/ 86Sr, magma, shielded from interaction with the wall-rock (Fig. 6). The small increase in 87Sr/ 86Sr observed laterally in equigranular syenite towards the pluton margin could represent diffusion-controlled isotopic exchange between the boundary magmas and adjacent portions of the interior syenite, rather than direct wall-rock assimilation. The model does not explicitly account, however, for the origin of the isotopic contrast between the interior and boundary portions of the pluton. One possibility is that the Ed,, = -5 boundary magmas were derived from the differentiation of a late mafic input into the magma system, which underplated the previously emplaced, lNd N - 2, magma from which the equigranular syenite crystallized. The origin of the isotopic contrast between the various magma
batches would then be related to differences in the magma source regions. Alternatively, mafic magma with isotopic compositions initially equivalent to that of the equigranular syenite could have assimilated wall-rock at the base of the system, concomittant with its differentiation, until an lNd = -5 was achieved, a process consistent with the higher magma and wall-rock temperatures at greater depths in the magma system [21] and the presence of restitic xenoliths of the Precambrian granite wall-rock in the inequigranular syenite. In this scenario, the equigranular syenite, inequigranular syenite, and the AFG could be derived from the same original magma batch. We note that due to the high Nd content (> 100 ppm; Table 1) and low lNd (- - 12) of the Precambrian granitic wall-rock, only 7% (Ma/M: = 0.07) bulk assimilation of this material by a 48% SiO,, lNd = - 2 magma is required to produce a 3 lNd unit decrease in the contaminated magma (but would produce only a small, N 1% SiO,, shift in the magma bulk composition). The proposed model also does not directly account for the apparent decoupling of the trace and major element compositions of the inequigranular syenite. Our preliminary data suggest that the high and variable LREE abundances could be controlled by variations in the abundances of accessory phases, particulary sphene, apatite and chevkinite. The fact that the REE and major elements abundances are not correlated in the border portions of the pluton, in contrast to the interior portions of the syenite, indicates that these accessory phases may be cumulate in origin. This conclusion is supported by resorption features observed in large apatite and sphene grains. Given that a sink for LREE is required to account for the observed decrease in LREE in the AFG, we consider it likely that the inequigranular syenite contains direct evidence that this decrease in REE was due to the removal of REE-rich accessory phases [13]. Detailed mineral chemistry studies are underway to fully assess these issues.
5. Conclusions Overall, our isotopic and chemical data suggest that the development of the observed compositional layering in the Organ Needle pluton involved bound-
P.L. Verplanck et al./Earth
and Planetary Science Letters 136 (199.5) 31-41
ary migration and ponding, at the top of the chamber, of buoyant magma isotopically distinct from the main exposed portions of the pluton. These results provide some of the first direct evidence that the production of silicic magma capping epizonal magma chambers does not involve the sidewall crystallization of the immediately underlying magma but is instead related to magma differentiation ( k wall-rock interaction) occuring closer to the base of the magma body.
Acknowledgements This work was supported by NSF grants EAR 91-05705 (Farmer) and EAR 91-06169 (McCurry). We thank Sam Seek (White Sands Missile Range) and Kevin Von Finger (Fort Bliss Military Reservation) for their help in obtaining access to the field area, and Gail Mahood and an anonymous reviewer for their editorial comments on the manuscript. [CL]
References [l] W. Hildreth, Gradients in silicic magma chambers: implications for lithospheric magmatism, J. Geophys. Res. 86, 10,153-10,192, 1981. [2] W.A. Duffield and J. Ruiz, Evidence for the revesal of gradients in the uppermost parts of silicic magma reservoirs, Geology 20, 111.5-1118, 1992. [3] A.R. McBirney, B.H. Baker and R.H. Nilson, Liquid fractionation. Part I: Basic principles and experimental simulations, J. Volcanol. Geotherm. Res. 24, l-24, 1985. [4] T.A. Vogel, D.C. Noble and L.W. Younker, Evolution of a chemically zoned magma body: Black Mountain Volcanic Center, southwestern Nevada, J. Geophys. Res. 94, 60416058, 1989. [5] F.J. Spera, D.A. Yuen, J.C. Greer and G. Sewell, Dynamics of magma withdrawal from stratified magma chambers, Geology 14, 723-726, 1986. [6] W.R. Seager and M. McCurry, The cogenetic Organ cauldron and batholith, southcentral New Mexico: Evolution of a large-volume ash-flow cauldron and its source magma chamber, J. Geophys. Res. 93, 4421-4433, 1988. [7] D. Butcher, Geochemistry and Nd/Sr systematics of selected lithologic units of the Oligocene Organ Batholith, south central New Mexico, 145 pp., Thesis, New Mexico State Univ., Las Cruces, NM, 1990. [8] W.C. McIntosh, J.F. Sutter, C.E. Chapin and L.L. Kedzie,
41
High-precision 4oAr/ 39Ar sanidine geochronology of ignimbrites in the Mogollon-Datil volcanic field, southwestern New Mexico, Bull. Volcanol. 52, 584-601, 1990. t91 D.P. Butcher, M. McCurry and G.L. Farmer, Evolution of the early Oligocene Organ cauldron, south central New Mexico, NM Bur. Mines Miner. Resour. Bull. 131, 35, 1989. [lOI K.C. Condie and A.J. Budding, Geology and geochemistry of Precambrian rocks, central and south-central New Mexico, 58, 1979. and cooling of the Pioneer [ill L.W. Snee, Emplacement batholith, southwestern Montana, 339 pp., Thesis, Ohio State Univ., Columbus, OH, 1982. t121 T.M. Harrison and I. McDougall, investigations of an intrusive contact, northwest Nelson, New Zealand, II: Diffusion of radiogenic and excess “‘Ar in hornblende revealed by 4oAr/ 39Ar age spectrum analysis, Geochim. Cosmochim. Acta 44, 2005-2020, 1980. [I31 C.M. Johnson, G.K. Czamanske and P.W. Lipman, Geochemistry of intrusive rocks associated with the Latir volcanic field, New Mexico, and contrasts between evolution of plutonic and volcanic rocks, Contrib. Mineral. Petrol. 103, 90-109, 1989. D41 H.E. Huppert and R.S.J. Sparks, The generation of granitic magmas by intrusion of basalt into the continental crust, J. Petrol. 29, 599-624, 1988. [151 C.M. Johnson, Isotopic zonations in silicic magma chambers, Geology 17, 1136-1139, 1989. [I61K.J. Tegtmeyer and G.L. Farmer, Nd isotopic gradients in upper crustal magma chambers: Evidence for in situ magmawall rock interaction, Geology 18, 5-9, 1990. [171 D.J. DePaolo, Trace element and isotopic effects of combined wallrock assimilation and fractional crystallization, Earth Planet. Sci. L&t. 53, 189-202, 1981. 1181G.A. Mahood and P.C. Comejo, Evidence for ascent of differentiated liquids in a silicic magma chamber found in a granitic pluton, Trans. R. Sot. Edinburgh Earth Sci. 83, 63-69, 1992. 1191E.D. Jackson, Primary textures and mineral associations in the ultramafic zoned of the Stillwater Complex Montana, US Geol. Surv. Prof. Pap. 358, 106 pp., 1961. DOI P.L. Verplanck, G.L. Farmer and M. McCurry, Sr and Nd isotopic constraints on the generation of large-scale compositional zoning in epizonal magma bodies, US Geol. Surv. Circ. 1107, 340 pp., 1994. Ml T.L. Grove, R.J. Kinzler, M.B. Baker, J.M. Donnelly-Dolan and C.F. Lesher, Assimilation of granite by basaltic magma at Burnt Lava flow, Medicine Lake volcano, northern California: Decoupling of heat and mass transfer, Contrib. Mineral. Petrol. 99, 320-343, 1988. t221 G.L. Farmer and D.J. DePaolo, Origin of Mesozoic and Tertiary granite in the western United States and implications for pre-Mesozoic crustal structure 2. Nd and Sr isotopic studies of unmineralized and Cu- and Mo-mineralized granite in the Precambrian craton, J. Geophys. Res. 89, 10,14110,160, 1984
Earth and Planetary Science Letters 136 (1995) 31-41
Isotopic evidence on the origin of compositional layering in an epizonal magma body P.L. Verplanck
a, G.L. Farmer ap*, M. McCurry b, S. Mertzman ‘, L.W. Snee d
a Cooperatiue Institute for Research in Environmental Science (CIRES) and Department ofGeological Sciences, Campus Box 216, University of Colorado, Boulder, CO 80309, USA b Department of Geology, Idaho State University, Pocatello, ID 83209, USA ’ Department of Geosciences, Franklin and Marshall College, Lancaster, PA 17604, USA d US Geological Survey, Denver, CO 80225, USA
Received 24 April 1995; accepted 31 July 1995
Abstract A detailed isotopic study of the Oligocene age (36 Ma), alkaline composition Organ Needle pluton in south-central New Mexico was undertaken to test models for the generation of compositional layering in silicic, epizonal magma bodies. The pluton is isotopically heterogeneous with its alkali feldspar granite composition cap (73-76% SiO,) having lower initial eNd and higher 87Sr/ ‘?jr ratio than the underlying main syenite (-5 vs. - 2 and - 0.709 vs. - 0.706, respectively). Both lithologies have isotopic compositions significantly different from those of the Precambrian granite wall-rock (Ed,, - - 12.1 and “Sr/ “Sr - 0.784 at 36 Ma). The isotope data indicate that none of the lithologies of the pluton represent the products solely of roof or wall-rock melting, and that the capping granite could not have been derived in a closed-system from differentiation of the underlying syenitic magma. However, the capping granite has isotopic compositions similar to those of a chemically heterogeneous inequigranular syenite found along the margin of the pluton at its deepest exposed level (4-6 km paleodepth). Field observations, and new 4oAr/ 39Ar age determinations, confirm that this lithology was comagmatic with the remainder of the pluton. We conclude that the capping granitic magma was derived from buoyant lNd - - 5 magma rising along the margins of the magma chamber and that the inequigranular syenite preserves a remnant of this sidewall magma. The main syenite body was encapsulated in this marginal, lower eNd, magma, while undergoing closed-system differentiation, most likely from a mafic progenitor. The Ed,, = -5 magma may represent mafic magma initially equivalent isotopically to the main syenite but which subsequently assimilated Precambrian wall-rock (- 10% by mass) at the base of the magma system.
1. Introduction The development of compositional zonations and/or layering is now generally accepted as com-
* Corresponding
author.
[email protected].
Fax:
(303)
492-1149;
email:
mon, if not ubiquitous, occurrence in silicic magma bodies [l]. Multiple processes are likely to be involved in the production of compositional heterogeneities in a given magma body, including melting of the magma chamber roof and/or wall rock [2], the upward movement and ponding of differentiated magma produced at the chamber base and/or sidewalls [3], and the sequential input into a shallow
0012-821X/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0012-821X(95)00147-6
32
P.L. Verplanck et al. /Earth and Planetary Science Letters 136 (1995) 31-41
magma chamber, and independent differentiation of, magma derived from greater depth [4]. Determining the relative importance of these processes for a particular magma body is hampered by the fact that only imperfect samples of the body can generally be obtained. Ash-flow tuffs may represent selective and incomplete sampling of the actual epizonal magma body [5] while exposed epizonal plutons suffer from late-stage differentiation or reequilibration [l]. Recently, however, it has been demonstrated that the Tertiary age, epizonal Organ Mountain batholith in south-central New Mexico contains at least one well-exposed pluton, the Organ Needle pluton (ONP), that has preserved both continuous and discontinuous compositional variations virtually identical to those inferred for pre-eruption magma bodies parental to many large volume, silicic ash-flow sheets [6]. As a result, the pluton represents a rare opportunity to study the origin of chemical layering and zonations in silicic magmas. In addition, because the Organ Needle batholith dominantly intrudes Precambrian granitic rocks which, from preliminary studies, are known to have Nd and Sr isotopic compositions markedly different from the Tertiary plutons [7], the ONP is ideally suited for studies of the origin of isotopic zonations in silicic magma bodies. In this study, we use combined elemental and Sr and Nd isotopic data to assess the origin of chemical and isotopic variations in the epizonal magma body from which the ONP crystallized.
2. Geologic setting The Late Oligocene Organ Mountain batholith is exposed in the Organ Mountains of south-central New Mexico (Fig. 1). The Organ Mountains represent a Late Tertiary fault block bounded by high-angle normal faults and tilted westward by 15-20“. Erosion of the fault block has exhumed a three-dimensional view of the epizonal intrusive rocks and the overlying Organ Mountain caldera with attendent silicic volcanic fill. Recent field, petrographic, and geochemical studies suggest that the youngest ashflow tuff, the tuff of Squaw Mountain, was deposited at N 36 Ma [81 and tapped the uppermost, highly siliceous portions of a shallow-level magma body represented by the ONP [6,9].
m
Younger Plutons
m
ONP SiO2 > 73 %
0
ONP SiO2 S&71% ONP SiO2 < 58 %
m
Precambrian Cranib
lb
_I
1kd h
Fig. 1. Geologic map of Organ Needle batholith (after Seager and McCurry [6]) showing spatial variations in whole-rock lNd values. The plus symbols in inset map delineate approximate area1 extent of inequigranular syenite. Qaf = Quatemary alluvium.
The ONP itself intruded near-surface and is now continuously exposed to a paleodepth of 4-6 km. At the structurally highest portions of the pluton, the ONP intrudes Precambrian granitic rocks, Paleozoic sedimentary rocks and, locally, its own volcanic cover, while at greater paleodepths it intrudes Precambrian (1.4-1.6 Ga) granitic rocks of the White Sands pluton ([lo]; Fig. 1). The ONP is alkaline in overall composition but shows marked compositional layering. The bulk of the pluton consists of syenite that is underlain by, and separated by a broad gradational contact from monzonite. The syenite is overlain by, and separated by a narrow compositional gradient from, an alkali feldspar granite (the intrusive equivalent of the Squaw Mountain t&f). The alkali feldspar granite (AFG) shows a narrow range of chemical compositions from 73 to 76% SiO,, but existing [6] and new chemical data (this study) reveal that the syenite is vertically zoned from N 57% SiO, to at least 68% SiO,. The continuous chemical zonation in the equigranular syenite is manifested by a general upward decrease in plagioclase anorthite
P.L. Verplanck et al. /Earth
and Planetary Science Letters 136 (1995) 31-41
content and the appearance of alkali feldspar, both as individual grains and as replacement of plagiclase, in rocks with SiO, > 65% [6]. Clinopyroxene (Ca-rich augite) is present in deeper and more mafic portions of the syenite, while hornblende and biotite occur over a much broader compositional range. Table 1 Isotopic and selected chemical SAMPLE
Si@l
While the chemical layering and zonation in the ONP were known from earlier studies, our new field observations along the easternmost, and structurally deepest, exposed portions of the batholith revealed that significant lateral compositional variations occur in the pluton as well. At this level in the pluton, we
data from the Organ Needle pluton
CaO
WA)
33
Wk)
K20
Na20
R@
(wt%) (wt%) @pm) @;I
87Sr/3 86Sr
B7Srf 4
Sm
Nd
86Sri
@Pm)
@Pm)
143Nd/5 144Nd
m4
ENd
W!lB!z jnea ui wan&r PV4391
57.0
4.77
3.84
5.05
78.5
676
0.70983+1
0.70966tl
16.6
94.6
0.512362+10
-4.97
PV5492 PV6491
58.3 60.5
3.91 3.31
4.75 4.29
5.18 5.75
80.2 65.6
626 1094
0.71006*1 0.70821+1
0.70987+1 0.70812~1
12.9 7.81
70.7 46.1
0.512338+9 0.51245tlO
-5.46 -3.23
PV4992 PV7591
61.4 61.6
2.65 2.43
5.29 5.57
5.56 5.63
98.3 101
3% 402
0.70936~1 0.7092&l
0.70899+1 0.70889+1
12.7 10.9
77.9 64.0
0.512386rlO 0.512367+10
-4.47 -4.86
PV6391
62.7
2.39
5.29
5.62
83.1
739
0.70824*1
0.70808tl
9.57
58.2
0.512394+10
-4.31
PV7291
63.2
1.68
6.27
5.39
107
314
0.71007+1
0.70957tl
22.4
94.2
0.512356+10
-5.26
PV5292 PV5091
63.6 65.6
2.20 2.40
5.54 4.66
5.22 4.67
115 135
347 380
0.70986+1 0.70729=1
0.70937+1 0.70677~1
12.9 9.14
77.7 53.4
0.51238+6 0.51245tlO
-4.59 -3.24
PV5092 :ouimonulm
67.0
1.03
5.90
5.49
144
155
0.7108422
0.7094723
16.6
108
0.512375~9
-4.65 -2.46
PV4392
62.8
2.32
5.23
6.28
63.9
741
0.70691+1
0.70678tl
7.60
46.2
0.51248929
PV6591
62.9
2.35
5.14
6.06
77.4
639
0.7066%1
0.70645tl
6.00
37.8
0.512494=10
-2.35
5309207 5309206
64.6 65.5
2.33 1.63
5.17 5.56
5.01 5.57
133 139
423 331
0.70585t2 0.7062Oil
0.70538i2 0.70557+1
10.2 8.44
59.6 49.4
0.512484+7 0.512473+7
-2.57 -2.79
5279202
66.6
1.63
5.41
5.31
214
291
0.70660*1
0.7055122
9.39
54.6
0.5125Olt7
-2.25
5279201
66.7
1.72
5.68
4.56
231
362
0.70635+1
0.7054Ot2
9.84
57.9
0.512491+7
-2.44
5299202
67.2
0.87
6.07
4.94
254
282
0.70701+2
0.70564~2
10.2
59.1
0.512471+10
-2.83
4lkali feldsorr
eranitp
PV35Yl
73.1
0.29
6.98
2.85
262
83.3
0.71453zl
0.7098Yz7
9.63
55.2
0.51235728
-5.06
PV3691
73.3
0.23
5.90
3.82
202
84.7
0.71447+1
0.71094+5
10.7
61.5
0.512365+8
-4.90
PV37Yl
73.5
0.23
6.72
2.98
250
100
0.71476+10
0.71105rlll
8.16
48.7
0.512354~10
-5.10
PV292
74.4
0.27
6.93
2.78
349
32.3
0.72905~2
0.71310+20
8.59
47.6
0.512358+9
-5.06
391
27.8
0.7360524
0.71537r30
5.00
31.1
0.51235~9
-5.16
PV192 76.1 0.36 5.97 3.16 P m U iscellaneous-mafic mclnw in ine6-iui9mnulnr PV6191 47.9 9.63 4vemae Premm6rian mnitic 68
2.2
1.18 rnck 5.0
suede
4.00
27.1
231
0.7154&l
0.71531
7.41
28.2
0.512525tlO
-2.03
3.3
238
189
0.78631
0.78398
21.7
110
0.511999
-12.1
’ Oxide concentrations by X-ray fluorescence at Franklin and Marshall College. Normalized to 100% volatile-free. ’ Trace element concentrations bij isotope dilution. Uncertainties are - 1% fot Rb, Sr and 0.5% for Sin, Nd. 3 *‘Sr/ s6Sr ratios were analyzed using 4-collector static mode measurements on a Finnigan MAT 261. Twenty-seven measurements of the SRM-987 standard during the study period yielded a mean 87Sr/ s6Sr = 0.71032 f 2 (2~ standard error). Measured s’Sr/ 86Sr ratios were corrected to SRM-987 of 0.71028. Errors are 20 of mean and refer to the last two digits of the 87Sr/ =Sr ratio. 4 Initial isotopic composition calculated at 36 Ma. lNd values calculated using 143Nd/ 144Nd (CHUR) = 0.512638. 2~ errors on Sr initial iostopic compositions represent combined errors on measured Sr isotopic compositions and on Rb and Sr concentrations. ’ 143Nd/ 144Nd ratios were normalized to 146Nd/ 144Nd = 0.7219. Analyses were dynamic mode, three collector measurements. Thirty-one measurements of the La Jolla Nd standard during the study period yielded a mean ‘43Nd/ ‘44Nd = 0.511838 + 8.
P.L. Verplanck et al. /Earth
34
1
and Planetary Science Letters 136 (1995) 31-41 400
n
Alkali feldspar granite
. . . .
300
....
n
-
0
0.0
syenite .
.
.
.
’ * . * . ’
CD,
.
.
.
n
r.g .., ~ ,1,
.
’ . . . .
+
8
200 -
.
. . . .
Rb
.
Equigranular syenite 0 (interior)
. . . .
I
* * . .
.
.
.
.
1
.
.
.
,
I
.
.
.
.
I
.
.
.
.
Sr
6.0
1000 -
S.o&
Na20
750 -
0
i 500
0
0
0
-
rot
l
250 -
I
0
Nd
100
.
80
m nw
60
n 40
0
w L
.
.
.
8
.
I
.
.
.
I
0% 00
.
.
.
.
I
E
.
.
Nd
.
.
, . . . .
20
:
:
I
.
.
.
.
I
.
.
.
.
I
87Sr /86Sr
.
.
.
.
n
0.7150
n
0.7125
I 0.7100
0. .
0.7075
55
65
70
7s
80
.
0
00
0.7050 60
l+ l
yq, .6.0 e
8
55
60
65
,
a0 70
SiO2
75
i 80
P.L. Verplanck et al. /Earth
and Planetary Science Letters 136 (199s) 31-41
identified a border lithology of the ONP that is exposed along a 200-400 m wide band directly adjacent to the wall-rock contact (Fig. 1). This lithology is an inequigranular syenite characterized by subhedral plagioclase phenocrysts partially to completely replaced by perthitic potassium feldspar set in a finer-grained matrix of quartz, potassium feldspar, hornblende, biotite and clinopyroxene and by variable and high abundances of accessory phases, particularly sphene and apatite. The inequigranular syenite contains rounded mafic enclaves, as does much of the pluton, and thermally metamorphosed, possibly restitic, xenoliths of disaggregated Precambrian mafic dikes and granite. Texturally, the transition between inequigranular and the coarse-grained equigranular syenite is gradational.
3. Analytical procedures and results We obtained Nd and Sr isotopic data, and Rb, Sr, Sm, and Nd concentrations (Table 1, Fig. 2) from multiple samples of the equigranular syenites, the capping alkali feldspar granite, and the inequigranular border syenite, to determine the relationships between these lithologies of the ONP. In addition, isotope and selected trace element data were obtained from six samples of the Precambrian wall-rock of the pluton, and from one mafic enclave from the inequigranular syenite. Initial (36 Ma) Nd and Sr isotopic compositions are referred to in the following text. Among the seven samples of the equigranular syenite analyzed, Sr abundances decrease (740-282 ppm), while Rb (64-254 ppm), Sm (6.0-10.2 ppm), and Nd (38-59 ppm) abundances increase with increasing SiO, (Fig. 2). Over the range of SiO, represented by these samples (63-67%) there are essentially no Nd isotopic variations ( lNd = - 2.3 to -2.8; Fig. 1, Fig. 2) and only a restricted range of 87Sr/86Sr (0.7054-0.7068). The two samples with 87Sr/ 86Sr > 0.7060 (PV4392 and PV6591) were obtained closest to the margins of the pluton (Fig. 4). Unlike the equigranular syenite there is no obvi-
Fig. 2. Selected major, trace and isotopic compositions
35
ous correspondence between chemical composition and geographic position in the capping AFG. However, over the range of SiO, sampled (73-76 wt%; Table l), the Sr and Rb concentrations continue to decrease (loo-28 ppm) and increase (262-391 ppm), respectively, relative to the most silicic portions of the underlying syenite. Both the Sm and Nd concentrations of the AFG decrease with increasing SiO, (11-5 ppm and 62-31 ppm). The Sr isotopic compositions of the AFG (s7Sr/ @jSr = 0.7099-0.7153) are significantly higher than the values determined for the equigranular syenite, with the highest 87Sr/ “Sr corresponding to the highest Rb/Sr, lowest Sr content samples (Fig. 4). The lNd values of the AFG are uniform (-4.9 to -5.2) and distinctly lower than the values determined for the equigranular syenite. Despite its limited geographic extent, the inequigranular syenite is grossly heterogeneous chemically (SiO, = 57-67%,) and trends towards more mafic compositions than the immediately adjacent portions of the equigranular syenite. Rb concentrations generally increase and Sr contents decrease (66-144 ppm, and 155-1094 ppm, respectively), with increasing SiO,. In contrast, the LREE abundances are not correlated with SiO, and are higher than those observed in other portions of the pluton (Sm = 7.8-22.4 ppm, Nd = 46-108 ppm). The inequigranular syenite shows a range of Sr and Nd isotopic compositions (87Sr/ 86Sr = 0.7068-0.7099, and cNd = -3.2 to -5.5), but the majority of the samples have Ed,, values I - 4.5 and 7Sr/ 86Sr 2 0.709. The one rounded mafic enclave analyzed from the inequi ranular syenite has low SiO, (47.9%) and 8F high Sr/ 86Sr (0.71548), but Ed,, (- 2.03) distinct from that of the enclosing syenite (Table 1). The surrounding Precambrian granitic rocks have significantly lower ENd values and higher 87Sr/ 86Sr ratios (average eNd - - 12.1, and 87Sr/ 86Sr - 0.784, at 36 Ma; Table 1) than those of any of the lithologies of the ONP. New 4oAr/ 39Ar age determinations were obtained to place age constraints on the inequigranular syenite. Biotite, potassium feldspar, and hornblende sepa-
vs. wt% SiO, in various lithologies
of the Organ Needle pluton.
36
P.L. Verplanck et al. /Earth
and Planetary Science Letters 136 (1995) 31-41
ployed to produce the 4oAr/ 3gAr incremental release spectra (Table 2; [ 111). The 4oAr/ 3gAr age determinations from hornblende, biotite, and potassium feldspar from the inequigranular syenite (sample PV5292) yielded
rates from a sample of the inequigranular syenite (PV5292) were irradiated (Table 2, Fig. 3) for this purpose. A sanidine separate of known age 181 from the Squaw Mountain tuff was analyzed as an intralaboratory comparison. Standard techniques were em-
Table 2 4oAr/ 39Ar analytical
data, Organ Mountain
Batholith,
New Mexico
29.0
RYIP __~
0.8!s4 1.0128
03425 0.4040
2.51 05057 2.49 13954 055917 250 l&lop 2.8705 1.15626 2.483 105LW 35014 1.40320 2.4% llav 2.9888 1.20201 2.4% llsop 35520 1.43fn5 2.483 124W 2.8626 1.14959 2.490 lvop 1.6141 0.64522 2.50 14X@ V529Xymite-lowerborderme-Potassiumfeldspar
9lXlP 55OP
IL%99
Es$F..~t~~~.~~* E !z lE3 1050 1100 1150
1.0203 EE 1:412a 15639 12108 1.9706 27286 4.4766 16.120
0.12926 0.64675 1.0143 :z ;mzE i.1949
3.386 1.578 1.685 1.752 1.8)’ Ez 2:m
22. 23. 23. 24. 25. 24. 27. 25. 73.
59.1 94.0 97.4 973 975 97.4 %.7 955 895 802 69.9
!I
49.8 83.7 74.8
E :;:
2’G 2.487
E 58.
Ez :z 4:1m 2 1350 ll”s 0.65333 5;: 4;. 1450 0.95816 0.38598 2.482 18. r’5292-Sy&1clower to&r zone-Biotile >tal as date: 33.91 t 0.19 Ma; plateaudate:34.78t 0.09Ma O.Cih&+ 0.25%; ;;vj6 mg 0.0350 0.69 19. 0.17649 0.1181 1.49 62. E 0.84472 0.37Bo8 2.234 129. 0.76W9 0.31451 2.427 139. E 0.52115 0.21w 2.48 113. loo0 0.52515 0.2117 2.48 95. 1050 0.90253 0.3620 2.49 15280 0.62flO6 2.433 E: 1lOOP 1.9039 0.77684 2.451 18. 115aP 1.6781 0.68485 2.450 8.2 13oop
1.0 $3 4.4 5.6 6.1 12.7 15.4 132 15.7 12.6 7.1
0.6 :3 2.7 3.8
E 912 87.8
:::
::;
:.“9
Zf fU.8 E3
7.2 24.1 69.2 79.4 79.0 75.0 77.3 79.5 88.0 90.4
i.; 32 1.9
::; 10.2 R.5 5.6 5.7 9.7 lb.9
20.9 18.4
365 t 05 36.0* 0.4 355 t 03 35.7* 02 35.47t 0.17 3553f 0.15 3535* 0.08 3553* 0.09 35.40* 0.07 353s t 0.08 35.4st 0.09 35.6f 0.2
47.48t 0.17 22.28l 0.12 23.78l 0.06 E::
: 8%
z:::: 30:73t 32.16t 33.02t 35.00* 33.65* 3431f 34.94t
0:10 0.06 0.06 0.05 0.0s 0.14 0.16
9.9t 1.1 21.3t 0.3 31.80t 0.0s 34.46t 0.10 35.3t 0.4 35.3* 0.5 3.55* 0.3 34.60t 0.07 34.8st 0.09 34.85t 0.10
.E’Ma:kochron qe: 35.6f 0.3Ma 0.33 0.53 0.70 1% 102.5 1050 1075 1100 112.5 11sOP 1175P
0.0075 00244 om37 0.0401 EG 055405 245% 20036 1.1680 1.4816
0.00&l 0.0136 0.0120 0.0173 0.0248 0.0440 !“9715’: 0.78825 0.45696 057937
E I!
2.523 2520 2.556 2.5%
::: 0.32
0.16 0.16 0.16 0.16 0.15
6.6 6.1 20.2 9.3 22.8 27.2 36.7 45.3 59.1 77.9 89.6 94.2 92.7 92.2
0.2 0.6 0.5 Lx 0.4 ::‘8 1.4 2.; 24.8 18.2 14.4
27.t 8. 33.* 2. 32.: 32 13.r2. 26.: 3. 2.8.*2. 33.* 3. 33.l 2. 34.6s1.4 36.60* 0.10 35.71* 0.10 35.67t 0.08 36.u t 0.25 36.18 r 0.19
P.L. Verplanck et al. /Earth
r-----1
r
200 r
r
10 I1
__--___+__ _I
_--__I
r _r - ’
r'
20
30
40 I
MI1
60
70 1
80
90
I
100
-AK, Released (%)
Fig. 3. 4oAr/39Ar the inequigranular
37
4. Discussion
,=--;1
r
-1
and Planetary Science Letters 136 (1995) 31-41
incremental release patterns for samples syenite and the Squaw Mountain tuff.
from
“‘Ar/ 39Ar plateau ages of 35.69 + 0.09 (1 a> Ma, 34.78 f 0.09 Ma, and no plateau, respectively (Fig. 3). Sanidine from the Squaw Mountain tuff yielded a plateau age of 35.46 + 0.14 Ma, which is virtually identical, within the specified errors, to the 35.75 f 0.12 Ma plateau age from the previously determined for this sample [8]. We interpret the hornblende plateau age as most closely representing the crystallization age of the inequigranular syenite, due to its high closure temperature for the retention of radiogenie Ar [12].
The large Sr and Nd isotopic contrasts between the pluton and the enclosing Precambrian granite (Table 1) preclude the possibility that any of the various intrusive lithologies, including the capping AFG, originated solely by partial or complete melting of Precambrian wall- or roof-rock. Instead, each portion of the pluton investigated, including the silitic cap, must have crystallized from magma injected into the upper crust that was derived from a source isotopically distinct from the felsic Precambrian crust. The ubiquitous presence of rounded mafic enclaves in the ONP, and the field evidence that the equigranular syenite was underlain by monzonitic magma in the pre-eruptive magma chamber, further suggest that mafic composition parental magma(s) were involved in the production of the ONP. A similar conclusion has been reached for many other Cenozoic silicic magma systems in the western United States [ 131. A direct derivation from mafic parental magma seems most likely for the equigranular syenite, given the similarity in the Nd isotopic composition of the equigranular syenite and the one mafic enclave analyzed (i.e., lNd N -2; Table 1). The uniform initial
Notes to Table 2: Mineral separates were prepared by magnetic separator, heavy liquids, and hand picked; grains were 60-120 mesh size (250-125 pm). Seperates were then hand picked to greater than 99% purity, cleaned in reagent-grade acetone and de-ionized water in an ultrasonic bath, air dried, wrapped in aluminum foil boats and sealed in silica vials along with monitor minerals prior to irradiation. Samples were irradiated at the TRIGA reactor at the US Geological Survey, Denver, CO. ’ Abundances of “Radiogenic 4oAr” and “K-derived 39Ar” are reported in V as measured on a Mass Analyzer Products 215 Rare Gas mol Ar per V of signal. Detection limit at the time of the mass spectrometer. Conversion to moles can be made using 9.736 X lo-l3 4oAr” and “K-derived 39Ar” are experiment was 2 X lo-t7 mol Ar measured on a Faraday detector. Analytical data for “Radiogenic caculated to five places: 4oAr/ 39Ark is calculated to three decimal places. “Radiogenic “OAr”, “K-derived 39Ar”, and “40Ar/ 39Ark” are rounded to significant figures using analytical precissions. Apparent ages and associate errors were calculated from unrounded analytical data and then rounded using associated errors. All analyses were done in the Argon Laboratory Geological Survey, Denver, CO. Decay constants are from Stieger and Jager [U]. The irradiation monitor MMhb-1 (1.555 wt% K), with an age of 520.4 Ma [16] was used to calculate J values for the analyses. ’ Argon isotopic composition is corrected for volume, mass fractionation, trap current, radioactive decay of 37Ar and irradiation produced 39Ar and interfering Ar isotopes. Production ratios measured on pure K,SO, and CaF, salts irradiated with the samples were used to correct for irradiation-produced aAr (from K) and 39Ar (from Ca). Corrections for Cl-derived 36Ar were determined using the method of Rodick [17]. Production ratios determined for all samples were: “ArsrJ 39Arx, 1.10 X 10W4; 38Ark/ 39Arx, 1.306 X lo-‘; 4oArk/39K, 7.8 X 10-3; of split gas fractions from each 39Arc,/ 37Arc,, 5.95 X 10m4; 36ArK/37Ca, 2.70 X 10m4; 3 Ark/ 3sArc,, 2.4 X lo-‘. The reproducibility monitor (0.15%, lo) were used to calculate imprecisions in J. J values were interpolated from adjacent monitors and have similar uncertainties to the monitor. Uncertainties in calculations for the date of the individual steps in a spectrum were calculated using equations yf Dahymple et al. [18]. To calculate apparent K/Ca ratios, divide the 39Ar/ 37Ar value by 2. p Fraction included in plateau date. l
P.L. Verplanck et al. /Earth and Planetary Science Letters 136 (1995) 31-41
38 0.716,
0.709lr
0
I
I
I
I
1
10
20
30
40
87Rb/86Sr
(36 Ma)
Fig. 4. *‘Sr/ “9 (36 Ma) vs. s7Rb/s6Sr for various lithologies of the Organ Needle pluton. Symbols as in Fig. 2. Inset diagram shows data for equigranular and inequigranular syenites-circled symbols are samples of the equigranular syenite closest to the pluton margin. Simple mixing and assimilation-fractional crystallization (AFC) modeling described in text. D values are bulk partition coefficients for Sr and Rb, and r is ratio of rate of assimilation to rate of crystallization. Tick marks mark variations in F, the fraction of magma remaining, and A4, /Mi, the ratio of the mass of assimilated crust relative to the original magma mass 1221. The simple mixing curve shown approximates the best-fit line through the AFG data, which corresponds to an apparent isochron age of * 10 Myr.
isotopic compositions of the equigranular syenite, in turn, implies that the range of bulk compositions observed within the interior, syenitic portions of the pluton was produced in a system closed to wall-rock interaction. The mechanism of magma differentiation that produced the range of bulk compositions in the interior portions of the pluton remains unresolved, although the major element variations in the equigranular syenite from 60 to 68% SiO, are mimicked by fractional crystallization of the observed mineral assemblages [6]. The relationships between the AFG and the inequigranular syenite, and either a mafic progenitor or the equigranular syenite are more obscure. An important observation from the AFG, for example, is that, unlike the equigranular syenite, significant variations exist in its initial Sr isotopic compositions. These isotopic variations are correlated with Rb/Sr ratios and yield an apparent isochron “age” of 10 Myr (Fig. 4). It is unlikely, however, that the AFG could represent an older intrusive body into which the magma parental to the equigranular syenite intruded,
as implied by the isochron age, given that the AFG itself intrudes the Cueva Tuff which has been dated at 36.2 Ma [8]. It is equally unlikely that the apparent isochron represents a magma residence age for the AFG, as it is unreasonable that the parental magma could have remained partially molten and in isotopic disequilibrium over 10 Myr even with recharge of basaltic magma to the base of the system [14]. Instead, the apparent isochron is most likely a byproduct of magma-wall rock interaction, a process not recorded in the isotopic compositions of the interior portions of the pluton. The correlation between increasing initial 87Sr/ 86Sr and decreasing Sr contents in the AFG is similar to that observed in many silicic ash-flow tuffs and is typically interpreted as evidence that magma-wall-rock interaction occurred through twocomponent mixing [15,16]. Simple mixing between the average White Sands pluton (Table 1) and a silicic magma with 87Sr/86Sr of 0.710, but which has developed a range of Sr concentrations (e.g., from 100 to 30 ppm) and Rb/Sr ratios, could generate a range of correlated Rb/Sr and initial 87Sr/ “Sr ratios similar to those observed in the AFG after only 1% bulk assimilation (by mass) of the wall-rock material. Concomittant assimilation and fractional crystallization (AFC; [17]) could also account for the correlated Sr isotopic compositions and Rb/Sr in the AFG, at low rates of assimilation relative to crystallization (r < 0.3). In this model, the range of initial isotopic compositions in the AFG represents variable amounts of assimilation, with samples having the highest “Sr/ 86Sr having assimilated the highest amount of crust relative to its initial magma mass (a maximum M,/Mz = 0.01 if the same initial magma and wall-rock compositions used above are assumed and using D,, = 2.5 and D,, = 0.5; Fig. 4). While the exact amounts of assimilated crust calculated in either AFC or simple mixing models obviously depends on the mechanism of assimilation (bulk vs. selective, for example), it remains valid that small amounts of wall-rock assimilation can account for the Sr isotopic variations, and for the uniform lNd values, of the AFG. In any case, removing the isotopic effects of the N 1% wall-rock interaction needed to account for the internal range of Sr isotopic compositions in the AFG still leaves a distinct Sr and Nd isotopic contrast between the AFG and
P.L. Verplanck et al./Earth
and Planetary Science Letters 136 (1995) 31-41
the equigranular syenite. This contrast implies that the AFG is not the end-product of the closed-system differentiation processes that produced the range of chemical compositions observed in the underlying equigranular syenite. Similarly, the Nd and Sr isotopic contrasts between the inequigranular and equigranular syenites preclude a simple closed-system relationship between these two lithologies, despite the gradational contact between the equigranular and inequigranular syenites and the 35.7 Ma ““Ar/39Ar hornblende plateau age for the latter, which imply that the magmas parental to both lithologies were contemporaneous. For example, the inequigranular syenite cannot represent the product solely of sidewall crystallization [3] of magma from which the interior syenite crystallized. It is also unlikely that the border lithology formed by mixing between magma isotopitally equivalent to the interior portions of the syenite and partial melts, restite, xenoliths, or xenocrysts derived from the immediately adjacent Precambrian granite wall-rock. While a range of major and trace element compositions for the inequigranular syenite might be produced by such mixing, a wide range of Sr and Nd isotopic compositions would also be generated. For example, simple mixing of bulk wall-rock and magma equivalent to the equigranular syenite produces a strong covariation between eNd and l/Nd (Fig. 5). A similar covariation would be
I oma
Omo
l/Nd
OMO
o.oM
(ppm)
Fig. 5. end (36 Ma) vs. l/Nd (ppm) for various lithologies of the ONP and for the Precambrian granite wall-rock. Ticked marked line is two-component mixing line between equigranular syenite (e Nd = -2.5) and average. Precambrian granite (boxed numbers are fraction of latter in mixture).
39
Fig. 6. Cartoon depicting model in which silicic cap of the Organ Needle pluton develops through accumulation of differentiated magma initially produced at the base of the magma system but which migrated upwards along the magma chamber walls. Numbers shown are eNd values for wall-rock, inputted mafic magma(s), and various lithologies of the ONP.
expected if the range of Nd contents in the inequigranular syenite were due to the mechanical incorporation of LREE-bearing accessory phases derived from the wall-rock. The observation is that the Nd isotopic composition of the border lithology is relatively uniform over a wide range of Nd concentrations (Fig. 5). The isotopic compositions of the inequigranular syenite (eNd = - 5, and 87Sr/ 86Sr N 0.709), however, are identical to those inferred for the magma parental to the AFG (prior to minor wall-rock interaction). Therefore, while neither lithology could have been derived from the equigranular syenite, the isotopic data allow for a direct genetic relationship between the AFG and inequigranular syenite, despite a spatial separation of at least 3 km in the original magma chamber. Such a relationship can be best accomodated in a model in which the development of the compositional layering observed in the ONP involved the upward migration of intermediate to silicic composition magmas along the margins of the magma chamber (Fig. 6; [18]). In such models, compositionally diverse magmas are produced by fractional crystallization near the base of an epizonal magma body [19] and then rise buoyantly along the
40
P.L. Verplanck et al. /Earth
and Planetary Science Letters 136 (1995) 31-41
chamber sidewalls, the most silicic magmas ( > 70% SiO,) reaching the top of the magma system [18]. We consider the inequigranular syenite to represent a remnant of the differentiated magmas that rose along the chamber sidewalls, and the AFG to represent the accumulation of silicic magma that traveled to the top of the chamber by this same route, athough we recognize that the exact disposition of various magma compositional types in the original magma chamber may have been modified during the eruption of the Squaw Mountain tuff. This model is attractive in that it accounts for the similar isotopic compositions of the AFG and inequigranular syenite and: (1) accounts for the wide range of bulk compositions observed in the inequigranular syenite over the restricted area over which it is exposed (- 1 km2), and the overall lack of field evidence for significant sidewall crystallization along the margins of the pluton. (2) is consistent with preliminary data from shallower structural positions in the pluton that reveal that lNd = -5 syenite is present, is restricted to the pluton margins, and increases in average silica content with decreasing depth [20]. (3) allows for the wall-rock interaction recorded in the AFG to have occurred during transit along the margins of the magma body, a process consistent with the occurrence of partially disaggregated Precambrian granite, in various stages of local bulk assimilation, along the margins of the ONP. The equigranular syenite is envisioned as chemically evolvin separately while encapsulated in lower A lNd, higher Sr/ 86Sr, magma, shielded from interaction with the wall-rock (Fig. 6). The small increase in 87Sr/ 86Sr observed laterally in equigranular syenite towards the pluton margin could represent diffusion-controlled isotopic exchange between the boundary magmas and adjacent portions of the interior syenite, rather than direct wall-rock assimilation. The model does not explicitly account, however, for the origin of the isotopic contrast between the interior and boundary portions of the pluton. One possibility is that the Ed,, = -5 boundary magmas were derived from the differentiation of a late mafic input into the magma system, which underplated the previously emplaced, lNd N - 2, magma from which the equigranular syenite crystallized. The origin of the isotopic contrast between the various magma
batches would then be related to differences in the magma source regions. Alternatively, mafic magma with isotopic compositions initially equivalent to that of the equigranular syenite could have assimilated wall-rock at the base of the system, concomittant with its differentiation, until an lNd = -5 was achieved, a process consistent with the higher magma and wall-rock temperatures at greater depths in the magma system [21] and the presence of restitic xenoliths of the Precambrian granite wall-rock in the inequigranular syenite. In this scenario, the equigranular syenite, inequigranular syenite, and the AFG could be derived from the same original magma batch. We note that due to the high Nd content (> 100 ppm; Table 1) and low lNd (- - 12) of the Precambrian granitic wall-rock, only 7% (Ma/M: = 0.07) bulk assimilation of this material by a 48% SiO,, lNd = - 2 magma is required to produce a 3 lNd unit decrease in the contaminated magma (but would produce only a small, N 1% SiO,, shift in the magma bulk composition). The proposed model also does not directly account for the apparent decoupling of the trace and major element compositions of the inequigranular syenite. Our preliminary data suggest that the high and variable LREE abundances could be controlled by variations in the abundances of accessory phases, particulary sphene, apatite and chevkinite. The fact that the REE and major elements abundances are not correlated in the border portions of the pluton, in contrast to the interior portions of the syenite, indicates that these accessory phases may be cumulate in origin. This conclusion is supported by resorption features observed in large apatite and sphene grains. Given that a sink for LREE is required to account for the observed decrease in LREE in the AFG, we consider it likely that the inequigranular syenite contains direct evidence that this decrease in REE was due to the removal of REE-rich accessory phases [13]. Detailed mineral chemistry studies are underway to fully assess these issues.
5. Conclusions Overall, our isotopic and chemical data suggest that the development of the observed compositional layering in the Organ Needle pluton involved bound-
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ary migration and ponding, at the top of the chamber, of buoyant magma isotopically distinct from the main exposed portions of the pluton. These results provide some of the first direct evidence that the production of silicic magma capping epizonal magma chambers does not involve the sidewall crystallization of the immediately underlying magma but is instead related to magma differentiation ( k wall-rock interaction) occuring closer to the base of the magma body.
Acknowledgements This work was supported by NSF grants EAR 91-05705 (Farmer) and EAR 91-06169 (McCurry). We thank Sam Seek (White Sands Missile Range) and Kevin Von Finger (Fort Bliss Military Reservation) for their help in obtaining access to the field area, and Gail Mahood and an anonymous reviewer for their editorial comments on the manuscript. [CL]
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