Contrasting 180 enrichment and origins of High ... - Science Direct

276

Earth and Planetary Science Letters, 65 (1983) 276-286 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

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Contrasting 180 enrichment and origins of High Himalayan and Transhimalayan intrusives P. Blattner 1, V. Dietrich 2 and A. Gansser 3 I New Zealand Geological Survey, P.O. Box 30368, Lower Hutt, and Institute of Nuclear Sciences, Private Bag, Lower Hutt (New Zealand) 2 Institutfi~r Kristallographie und Petrographie, ETH- Zentrum, CH- 8092 Zi~rich (Switzerland) 3 Geologisches Institut, ETH-Zentrum, CH-8092 Ziirich (Switzerland)

Received February 15, 1983 Revised version accepted July 28, 1983

Twelve analysed leucogranitesof the High Himalaya in Bhutan (Chung La, Mrnlakarchung) and Garhwal (Badrinath) are among the most 1SO-enrichedgranites known (11.5-12.4%~818OsMow with two exceptions) and separate minerals show good isotopic concordance. The data strongly support an origin of the granites by anatexis of continental basement such as the Indian crystalline basement sheet or slab, undercut by the Main Central Thrust, of which five samples were analysed. In contrast, the pre-collision Transhimalayan (Gangdese) batholiths to the north of the Indus-Tsangpo suture, as exemplified by the Ladakh intrusives, show an initially oceanic trend of 8180 vs. SiO2 that becomes gradually somewhat enriched with respect to Hachijo-Jima. While not completely outside the range of enrichment that seems possible by fractional crystallisation, this could tie in with the 87Sr inhomogeneitiesreported by Honegger et al. [9], which may be due to assimilation of variably radiogenic Eurasian continental basement. For both the leucogranites and Ladakh intrusives the lSo levels and the concordance between minerals rule out significant cumulative water/rock ratios in syn- or post-magmatic interaction with subsurface waters.

1. Introduction The tectonic a n d chemical difference between the extensive calc-alkaline T r a n s h i m a l a y a n batholiths and the leucogranites of the High Himalaya, which occur equally o n the scale of batholiths (Fig. 1), has been c o m m e n t e d o n as early as 1907 [1] a n d was set out in more detail b y G a n s s e r [2,3], L e F o r t [4], Jin Chengwei a n d Z h o u Y u n s h e n g h [5], a n d others. Sr isotope data have already been reported for b o t h types of intrusives a n d high initial 87Sr/86Sr a n d low initial 143Nd/144Nd ratios provided the best evidence so far for a crustal origin of the leucogranites [6-10]. Here we are presenting some of the first systematic oxygen isotope analyses of the two rock series. I n some ways these support earlier interpretations, b u t due to the high a b u n d a n c e ratio of oxygen in water 0012-821X/83/$03.00

© 1983 Elsevier Science Publishers B.V.

a n d oxygen in silicates, oxygen isotope data provide i m p o r t a n t constraints on water-rock interaction scenarios.

2. Analysis and results Total rock samples of 5 - 2 5 g were g r o u n d to 6 0 - 4 0 0 /~m grain size, or less in the case of olivine-bearing rocks, a n d minerals separated by h a n d picking. Samples were reacted with b r o m i n e pentafluoride, a n d analysed b y procedures somewhat modified from C l a y t o n a n d M a y e d a [11]. Results are reported as %o 8~8OsMow b u t are tied more directly to the value for NBS-28 quoted in T a b l e 1 (9.80) a n d ol,r own laboratory standard. The tie to NBS-28 is useful, because of existing i n t e r l a b o r a t o r y differences of several tenths of a

277

T S 1 Transhimalayan Batholiths Leucogranites

~

Suture Zone with ophiolites

Lel" '~

.

/ 1

,

I

I

500 km

h t

.300

IDelhi

Y

'

80o

I

goo

Fig. 1. Tectonic sketchmap of the Himalaya area showing relative position of plutonic units studied. MCT = Main Central Thrust. M B T = Main Boundary Thrust. The Indus-Tsangpo suture consists of ultramafic mantle material and ophiolite m61ange, associated with Mesozoic pelagic sediments. The Indian crystalline basement sheet extends from the MCT to the suture, but is overlain by Tethyan sediments approximately north of the line of leucogranites. Leucogranites shown are from west to east: 1 = Badrinath, 2 = Api, 3 = Mustang, 4 = Manaslu, 5 = Shisha Pangama, 6 = Makalu, 7 = Chung La, 8 = M6nlakarchung.

permille. Care was taken to obtain 100% oxygen yields, b o t h for samples and standards. The following definitions are used, with R = 1 8 0 / 1 6 0 : 8 1 8 0 ( ~ ) = [ ( R s a m p l e / R s M o w ) - 11103 etl-2 = R 1 / R 2 ( " fractionation factor")

A1.2 = 103 In cq. 2 --- 61

-

~2"

The subscripts 1 and 2 refer to different crystal phases or liquid magma. All results, including analyses of 19 minerals, are given in Table 1.

3. Leucogranites In northern B h u t a n typical H i m a l a y a n leucogranites, often with accessory tourmaline and garnet, occur widely scattered as well as in three major centres, i.e. C h u n g La in the northwest, C o p h u La, in the north, and M6nlakarchungPasalum in the northeast. They have been dated as approximately 20 m.y. old [8]. The larger granites

are c o m m o n l y surrounded by networks of small stocks, sills, and dikes, and some large bodies of several kilometres thickness resemble laccoliths and sills. The country rocks consist of Precambrian gneissic and migmatite basement, Precambrian metasediments, and Tethyan metasediments roughly in northward dipping succession (Fig. 2). The granite samples represent the main rock types, rather than contact zones, and are from C h u n g La [2], where the leucogranites intrude the Tethyan series, M6nlakarchung [3], G o p h u La [1], and from a sill of about 10 km length, northwest of T h i m p h u [1]. In addition, we analysed five samples of Precambrian gneissic basement from the area between the Main Central Thrust and the exposed leucogranites, because this zone is likely to extend into the base of the leucogranite p l u t o n s (Himalayan gneiss, crystalline sheets of Indian Shield basement, cf. profile in Dietrich and Gansser [8]; also " T i b e t a n slab" of LeFort [4,12]). To b r o a d e n the data base, five leuco- and aplitoid granites from the Badrinath intrusion in Garhwal

278 TABLE 1 Oxygen isotope analyses of Himalayan intrusive rocks and basement (Bhutan, Garhwal, Ladakh) 81SOsMow (%O)

SiO 2 (%) [8,9]

Bhutan basement GH

54

coarse quartzofeldspathic two-mica gneiss (g) a feldspar

GH GH GH GH

300 | 384 / 418 544

coarse quartzofeldspathic two-mica gneiss

(g) (g) (g) (without g) quartz

10.1 b 9.6+0.1 (2) 8.9 ± 0.2 (2) 9.3 10.2 ± 0.2 (2) 10.5 b 11.2

Bhutan leucogranites GH

89

muscovite leucogranite (g, t) a quartz feldspar muscovite

GH

93

muscovite leucogranite (t) muscovite leucogranite (t)

73.16 73.13

muscovite leucogranite (t) muscovite leucogranite (t) muscovite leucogranite (t) aplitoid granite (t) aplitoid granite (t)

11.8±0.2 11.5 11.5 11.9±0.1(2) 12.1

74.40 73.27 70.54 73.77 76.20

muscovite leucogranite (t) feldspar

GH 217

muscovite leucogranite quartz feldspar

GH 221

muscovite leucogranite (t) quartz

G H 471 GH 513

74.07

&3 12.2 13.4 11.1 12.2 11.6 12.1 13.0 ± 42 (2) 148 12.4 13.6 10.3±0.1(2) 11.9±0.2(2)

muscovite leucogranite (t) quartz feldspar

G H 129

9.5 11.2

71.19

72.54 73.20

72.48

Badrinath leucogranites G G G G G

176 177 178 179a 179b

Ladakh intrusioes (1) Kargil- Mt. Somau HT 124 olivine-gabbro (cumulate) HT 125 HT 129 gabbronorite feldspar HT 132

quartz-diorite (cumulate) feldspar

HT 133 HT 135

quartz-diorite (cumulate) granodiorite quartz feldspar

(2) Stagma-Shey HT 161 HT 162 HT 157

microdiorite xenolith hornblende-diorite quartz-diorite

6.0±0.15 (2) 6.0 ± 0.1 (3) 6.0 b 6.4 ± 0.2 (3) 6.7 ± 0.2 (3) 7.3 7.1 ± 0.2 (3) 7.6 b 9.0 7.1

39.80 42.30 45.47

5.5±0.2(2)

49.21 53.28 57.89

6.4±0.2(3) 6.8

60.89 64.37 72.10

279 T A B L E 1 (continued)

HT

HT

HT HT

164

granodiorite

144

SiO 2 (%) [8,9]

7.2

67.40

quartz feldspar

8.1 6. 8

quartz feldspar

8. 7 7. 0

biotite-granite

166 159

~I8OsMo w (c~)

7.4 b

biotite-leucogranite aplitoid granite

70.92

7.4+0.1 (3) 7.4

Reference NBS 28

73.75 76.02

9.80

g = garnet, t = tourmaline. b Total rock result calculated from mineral analyses modes [9], and estimated fractionation factors.

14

12

g"

0

~

l

l

25

G

¢o

±

10 E] 8

{}

Leucogranites

Gneiss

(Fig. 3) have also been analysed. They also represent examples from the interior of the granites at least about 1 km from contacts. The illustrations phot. 11, 12 and 91 of Gansser [2] show typical field settings (Satopanth V., Shivling, M6nlakarchung). Fig. 2 shows isotope data and sample localities for the Bhutan granite and gneiss sampies, and the Badrinath granites. In the several test cases, the isotopic concordance between quartz, feldspar, and muscovite of leucogranites and basement gneiss is good. Oxygen

BP BadrlnathP~k 714Orll × x

x x x x

Fig. 2. Oxygen isotope composition and geological setting of leucogranite and basement gneiss samples in Bhutan. Circles = total rocks (empty circles estimated from minerals), triangles = ~luartz, squares = feldspar, rhomb = muscovite; G = field for five leucogranites of the Badrinath intrusion. LeFort's [12] preliminary data for Manaslu range from 11 to 12.5%o. The bar graph represents 506 180 analyses of granitoid rocks from Taylor [17], O'Neil and Chappell [18] (S- and I-types), Masi et al. [19], Lee et al. [20], H a r m o n and Halliday [21] and Matsuhisa et al. [22]. M a p abbreviations: M C T = Main Central Thrust, M B T = Main Boundary Thrust, L G = leucogranites, T S =

B G S

~ ~ x

Badrinath

BhagathKharakglacier Gangotriglacier Satopanthglacier

l

~

High Himolo~n crystalline

[~

LesserHimalay~

Fig. 3. Badrinath leucogranite and sample localities. M C T = Main Central Thrust.

Tethyan sediments, p C G = gneiss and mlgmatite of crystalline basement sheet (Precambrian; north of M C T only), p C S = metasediments of basement sheet. C = Chomolhari, K = Kunla Kangri, G = Gangtok, P = Paro, T = Thimphu.

280 isotope fractionation between quartz and alkalifeldspar (mircocline and albite) are 2.1, 2.2 and 2.3. These are about average values for granites, and typically give somewhat low temperatures of less than 600°C, by any of the available thermometric calibrations [13]. Minor feldspar alteration evident in thin section may have required the presence of only minute quantities of water, and the level of 180 enrichment, as well as the concordance between minerals, show that we are looking at 6180 values very close to the original ones and therefore at retrogression, if any, in an almost closed system with a low w a t e r / r o c k ratio. Previous workers, using several geochemical lines of evidence, have already suggested the Himalayan leucogranites to have formed by partial or complete anatexis [4-8,10,12,14]. In contrast to Sr and Nd, oxygen isotope ratios of phases in equilibrium are known to differ considerably, probably at temperatures up to 1000°C [15]. In particular, granitoid partial melts are concentrating the most 180-enriched fraction of a rock in the form of SiO 2, H20, and alkali-feldspar. Depending on rock type and the extent of anatexis, the 6180 of melts may therefore be equal to or up to some 0.57~ larger than that of the starting material. In the case of Bhutan the gneiss basement has moderately high 8180 values between 9 and 11%o. Thus, granites G H 89 and G H 471 could directly represent partial melts of a basement gneiss sheet similar to that sampled further south, and still lower 180 values would severely constrain partial melting models. However, metamorphosed Precambrian and Paleozoic pelitic and carbonate-rich sediments intercalated with, and overlying, the basement gneiss [8,12] are certain to have more enriched values, nearer 15%o. The relatively high 8180 near 12.1 and 11.8%o, respectively, of most of the Bhutan and all of ,the Garhwal examples is readily explained, therefore, by the presence of mixed basement containing metasediments, in the zones of anatexis. These zones must lie considerably northward and downdip from those parts of the crystalline sheets that now underlie the intrusions (cf. [8 fig. 12] and [12, fig. 6]). Although the prominence of marble in metasediments of the intruded areas, and its contact metamorphism, might suggest some form of

oxygen isotope exchange in the gas phase, the petrographic uniformity of the leucogranites and the apparent upper cut-off near 12.5%~, argue strongly against a major such effect in the zone of emplacement. An influence of subcrustal or oceanic magma on leucogranite genesis would of course be difficult to uphold because sufficient contamination by material near 12%o to raise the 6180 of a magma with < 8goo to 12%o would lead to its solidification long before completion of the process [16].

3.1 World-wide comparison To construct a valid world-wide frequency distribution of oxygen isotope compositions of "granites" is difficult, because the literature provides, necessarily, very uneven background information as to petrography, types of geological setting, and volumes of rock represented by given analytical values. However, as a beginning 506 analyses (not including quartzdiorites) are summarised at the top right of Fig. 2. They could approximate an unbiased sample of granites and granodiorites in general, and are from Taylor [17], O'Neil and Chappell [18], Masi et al. [19], Lee et al. [20], Harmon and Halliday [21], and Matsuhisa et al. [22]. Only eight results (1.6%) are over 12%o. Several granitoid plutons emplaced in high-180 pelitic metasediments, and studied in detail by Shieh and Taylor [23] reach values above 11%o only within 250 m or less of intrusive contacts. While it is clear that the Bhutan and Garhwal leucogranites, as well as the preliminary examples from Nepal cited by LeFort [12] (11-12.5%o), are among the most 180-enriched granites anywhere, this does not imply that they are unique. Searching especially for high-180 granites, one finds examples in part of the Cretaceous to Tertiary "Ilmenite Series" granites of Honshu [22,24] (10-12.5%o), in a part of the Variscan Maladeta Complex of the Pyrenees [25] (11.3-11.8%o) and in the most acidic members of the Jurassic Snake Creek intrusive of the Basin and Range Province [26] (11.2-12.27~). The Devonian peraluminous South Mountain batholith of Nova Scotia shows a range of 10-12%o [27] and the Hercynian Cornubian batholith of England one of 10.8-13.2%o [28]. These last two batholiths commonly carry such minerals as

281

andalusite, garnet, topaz, and tourmaline, and because of this and their homogeneity, they provide perhaps the best ancient analogues of the Himalayan leucogranites. It is noted that no sizeable granite bodies with 8180 greater than about 13%o seem to be known as yet, in spite of the fact that clays such as used by Winkler and von Platen (e.g. [29]) in experimental anatexis, as well as pelitic schists, commonly have ~180 values higher than that (cf. section 5.3).

4. Ladakh intrusives

The Indus-Tsangpo suture is considered to be a complex expression of the collision boundary between the continent of India and the elements of northern Eurasia [3,30]. On its northern side, the suture is accompanied over a distance of more than 2000 km by the parallel belt of the Transhimalayan or Gangdese batholiths. The several lined-up batholithic bodies (Fig. 1) have been emplaced on the southern continental margin of Eurasia in advance of its collision with the Indian craton [3,5,9] and resemble their Andean counterparts [30]. Their emplacement may have spanned a period of about 70 m.y., at least from 103 to 39 m.y. ago [9]. The Ladakh intrusioes are part of this calc-alkaline belt and were described by Frank et al. [31] and Honegger et al. [9] as multiple intrusions into the Jurassic to Upper Cretaceous Dras volcanics. They consist of about the following proportions of differentiates: 15% mafic, 25% intermediate, 50% granodiorite, and 10% granite, and have a cortege of predominantly rhyolitic volcanics, exposed to the north and east. The analysed samples are from two sections, (1) Kargil-Mt. Somau and (2) Stagma-Shey (near Leh) [9,31]. Fig. 4 shows the oxygen isotope data plotted against wt. % SiO 2. The two sets of samples define very similar trends, and only a combined regression line is justified. The concordance for independent analysis of quartz [3] and feldspar [5] is again good, with A (quartz-feldspar) values 1.3, 1.7, and 1.9 indicating higher temperatures, or appearing less retrogressed, than in the case of the leucogranites (Table 1). The behaviour of oxygen isotopes in magmatic

I

~o ~z

z 11 t/1 0 10

I

o Gorhwol ~ • Bhufan J Leut0gronifes 0

6O 9 i 0

I

o• Kargit 1 Sfngma-Shey Transhimotaya

13

Field for Taupo I HochiJ0-]imo

I

o~o~ °

~ ,~"N~

"

8

o~ 7 6 I

t~0

°l

50

I

60 % Si02

I

70

Fig. 4. 8180 vs. SiO 2 trend of Ladakh intrusives and values for leucogranites. Shown for comparison is the primitive HachijoJima trend [32] with a = 0.9995, after a small adjustment to NBS 28 = 9.8, and the contaminated and highly dispersed T a u p o I trend [15]. The solid line for Ladakh has a regression coefficient of 0.93. The Darran data plot close to the lowest value for Stagma-Shey.

differentiation is dominated by the generally increasing level of ~80 (as well as SiOz and incompatible elements) of minerals crystallising successively. Iron discriminates most strongly against 180 in its compounds, and the timing and quantity of iron precipitation are important superimposed factors for the ~80 trend of a given sequence. The 8180 values of granitic residues are typically about 1%o higher than those of corresponding early basaltic liquids, but the difference could vary between 0 and 29~ independently of secondary contamination [15]. As an actual example, the Hachijo-Jima sequence of the Izu-Mariana arc, which is unlikely to have assimilated oxygen from continental sources during differentiation, shows an overall enrichment of 1%o. In this case, with " x " and "1" referring to crystals and liquid, respectively: ax_ l = 8x_ll0 -3 + 1 = 0.9995 or Ax4 --- - 0 . 5 ~ , with a approximately constant [32] (dashed line in Fig. 4). In another oceanic setting Muehlenbachs and Byerly [33] report an abrupt change of ax4 from 0.9999 to 0.9990 at that point of a sequence from the Galapagos spreading center, where SiO z --- 52%, and substantial titanomagnetite begins to

282

crystallise. Note that in plotting 8a80 against a chemical fractionation index such a s S i O 2 rather than percent of initial liquid crystallised, there will inevitably be additional scattering, since fractionation factors for both variables may change. The liquid and cumulate tracks are unlikely to be identical or mere extensions of each other, although cross-contamination of liquids and cumulates in real samples would dampen the possible excursions. Contamination of the original magma, or evolving differentiates, by assimilated country rock is a second and often dominant cause of 180 enrichment and this process, apart from Rb decay, is often the major cause of 87Sr enrichment. Contamination usually leads to enrichment rather than depletion in a80, because differentiating magmas tend to start out near mantle compositions, at or just below 6%o 8180, whereas most of the possible host rocks are substantially higher in 180, common values for sediments being 10-20%o. The contamination of Andean-type batholiths to some degree represents the clash and mixing of dissimilar oxygen and strontium isotope ratios in continental accretion. Two extreme situations are seen as follows: (1) The original magma already contains a crustal component, or is homogeneously contaminated at the source only. In that case the differentiation trend would start at an enriched level, but maintain a "normal" slope, with a = 0.9995 + 0.0005 for 180/160, and produce a true Rb-Sr isochron, with merely a raised initial 87Sr/86Sr, and a level plot of (87Sr/86Sr)i. An example may be the Batopilas ignimbrites studied by Lanphere et al. [34]. (2) The contamination is secondary and its degree is partly a function of the opportunity and time available for the evolving magma to assimilate country rock. For example, if increasingly acidic fractions became increasingly contaminated, the closed system fractionation line would be of steeper slope. Examples are the Taupo I trend [15] (also shown in Fig. 4) and several on-land Japanese trends of Matsuhisa [32]. This process would produce a pseudo Rb-Sr isochron (Taupo andesites [35,15]). Other processes, including a novel one recently

proposed by Morse [36], could produce 87Sr trends similar to those under (2) without or with modified assimilation of country rock, and may need to be considered when more data become available. The 8180 trend for the Ladakh intrusives in Fig. 4 is steeper than the Hachijo-Jima line, but has approximately the same initial 8180 value. If ascribed only to secondary contamination the increased slope of the Ladakh trend could mean, say at 65% SiO2, admixture of 13% of a Eurasian orthogneiss basement with 10%o 8180 or 8% of a mixed gneiss/metasediment basement with 12%o 8180. However, the uncertainty in the averaged ax_l value of a differentiation sequence means that we cannot prove this degree of contamination. Matsuhisa [32] and Blattner and Reid [15] found assimilation to be highly selective, leading to considerable dispersion of 8180 trends at the volcanic level, and variations in chemical Sr contents must produce additional dispersion in 87Sr as opposed to 180 trends. The plutonic Ladakh sequence has in fact a 8180 dispersion almost small enough to correspond to Rayleigh fractionation with constant fractionation factors; the exception of sample H T 161, with about normal Fe contents, could represent an isolated residue. On the other hand, the data of Honegger et al. [9] show considerable and apparently unsystematic dispersion of 87Sr/86Sr for the lowest-Rb examples as well as the whole Rb-Sr evolution diagram of Kargil/ Stagma-Shey. These variations are not expressly assigned to crustal contamination by Honegger et al., although, since Sr isotopes show negligible partitioning between phases, they could provide the discriminating evidence for such contamination that the 8180 variations here are to small to provide.

5. Discussion

The difference in oxygen isotope composition between Himalayan leucogranites of Bhutan and Garhwal and the Transhimalayan batholith in Ladakh is consistent and striking. In spite of their relatively large volume, the leucogranites are among the most a80-enriched of over 500 analysed granites world-wide; other granites the2¢ resemble

283 perhaps most are certain Upper Paleozoic S-type batholiths (South Mountain, Nova Scotia [27]; Cornwall, England [28]). With 6.0-7.6%o 8180 the Ladakh intrusives, on the other hand, are only about 0.3%o enriched relative to magmatic differentiates from oceanic environments.

5.1 REE and lack of 1So shifts of leucogranites There seems to exist broad agreement between workers using very different approaches, on the likelihood that the leucogranites originated from fusion, at 20 km or more depth, of the crystalline basement sheet or slab, which is underthrust by the Lesser Himalayas on the Main Central Thrust [4-8,10,12,14]. Vidal et al. [37] maintain a similar view, which is well supported by Sr isotopes, but find it difficult to accommodate a large and quite variable REE depletion of the leucogranites relative to the "Tibetan slab" in Nepal. This depletion is well matched by the REE data of Dietrich and Gansser [8] from Bhutan. Of several explanations proposed by Vidal et al. one involves a model of hydrothermal exchange of the melts in a watersaturated environment with a twenty- to seventyfold cumulative mass excess of REE-free surface water. For this kind of interaction between water and silicate, oxygen isotopes provide an ideal complementary test: because of the high oxygen content of water relative to silicates, the exchange, which amounts to leaching as far as the REE are concerned, would have completely replaced the original oxygen of the granites, bringing it into isotopic equilibrium with the water [38,39]. In order to maintain the actual high 8180 values of the leucogranites, these required enormous amounts of surface water would have had to be isotopically shifted into equilibrium with crustal rocks before reaching the granites of their melts, thus denying the direct access of REE-free surface waters envisaged by Vidal et al., and also implying a .trail of low-180 crustal rocks an order of magnitude larger than the volume of the granites themselves. The solutions causing such a trail would have reached compatibility with the gneiss basement with regard to REE much more rapidly than they can have with regard to 180/160, because of the very low potential "consumption" of REE by the water.

It seems more likely, therefore, that one of the other options offered in Vidal et al. [37] should apply. In particular, we are not convinced that the REE, which reside largely in accessory and refractory minerals, should be concentrated in ultrametamorphic melts, merely from analogy with their "incompatibility" in the high-temperature environments of the petrologically different upper mantle and subduction zones. Vidal et al. share these doubts [37, p. 2299]. The REE may have remained in the mafic residues of partial melts and only a small fraction of the residues may have been incorporated in the segregating melts. We note by the way that specimen G H 89 which has the lowest 8180 of the present leucogranites, also is by far the lowest in LREE of four examples of Dietrich and Gansser [8] and the second lowest of a combined eleven examples if those of Vidal et al. [37] are included. However, sample G H 93, which is also from the Chung La centre, has a high 8180, suggesting lack of lateral mixing among the ascending anatectic melts, as well as source inhomogeneity. A similar picture is put forward by Ferrara et al. [40] for the Nuptse granite in Nepal on the basis of inhomogeneous Sr isotope ratios (also cf. [8]). The other relatively low-180 leucogranite, G H 471, is one of the most LREEenriched leucogranites of Dietrich and Gansser [8] or Vidal et al. [37], so that no good correlation between LREE and 180 may exist. Mehnert [41,42, with English summaries] provides well documented studies of the petrographic aspects of anatexis and palingenesis of Variscan migmatites of the Black Forest (Germany) with regard to major and minor minerals, which may be of some significance for the genesis of the present leucogranites.

5.2. Comparative 180/160 and (SZSr/arSr)i of Ladakh intrusives and other differentiation sequences Data of potential relevance to the question of crustal contamination are given in Table 2. Results on calc-alkaline leucogabbros of the Darran Complex and metamorphic equivalents are included, because of the low 8180 relative to Hachijo-Jima, in view of uniform standardisation, and low abso-

284 TABLE 2 Oxygen and strontium isotope data for some oceanic magmatites at SiO 2 = 50% Hachijo-Jima

Darran

Ladakh

81SO (%o)

6.0 [31]

5.5-5.7 [44]

6.2 this paper

(STSr/86Sr)initial

0.7032 - 0.7038 [43]

0.7036 - 0.7038 [44]

0.7041 - 0.7053 [91

lute 8180 in spite of a relatively high reference value for NBS 28. These leucogabbros have well developed coarse cumulate textures, and are part of batholithic intrusions into a pre-Mesozoic continental margin near the west coast of present-day New Zealand [45,46]; petrographically they greatly resemble some of the Ladakh rocks. If liquids corresponding to the Darran cumulates had been similar to Hachijo-Jima, their relative enrichment in a80 and SiO 2 might not have produced, in any simple way, an overall trend similar to HachijoJima. Alternatively, therefore, the data of Table 2 suggest that both the Hachijo-Jima and Ladakh magmas may have been contaminated by 'crustal material. For the Ladakh sequence in particular, both oxygen and strontium isotopes suggest initial contamination (of type 1, section 3) to be present. Considering now the scattered 87Sr data of Honegger et al. [9] to be due purely to secondary crustal contamination, of type 2, we have seen that for oxygen, near 65% SiO 2. admixture of 8-13% of material with 12 to 10%o 8180 could be postulated, being "optional" so to say, as long as aggregate fractionation factors are known only to the nearest 0.5%o. In contrast, the high dispersion of initial 87Sr/86Sr ratios [9] could suggest the existence of high-87Sr (and variably low-common Sr) contaminants. To the south of the Indus-Tsangpo suture, intrusives with 8 7 8 r / / 8 6 S r ratios of 0.75 or more occur widely in Indian shield basement of ca. 500 and 1800 m.y. age respectively [8,40], and the same may be true for Eurasian basement at the time of the Transhimalayan intrusions. Assuming roughly equal Sr percentages in magma and contaminants, 8% of such material would raise an initial 87Sr/86Sr of 0.7040 of a Ladakh magma to 0.707. Smaller,

variable amounts of contaminants could therefore account well for the scatter of initial 87Sr/86Sr values, without requiring a greater than observed dispersal of the 8180-SIO2 line, which could be based on a pre-contamination a value close to that for Hachijo-Jima. The tempting "higher-order" models presented by DePaolo [47] may bring with them added complications, such as the need of not only thermal, but concurrent chemical equilibration or mixing, in a magma chamber. In the present case important factors, including the nature of the deep basement of the Transhimalayan intrusives and their homogeneity with respect to age, are only partly known, and more detailed field studies as well as combined 180-87Sr measurements seem desirable. Preliminary data for the Gangdese section of the Transhimalaya in Xizang Province show a 8180 trend very similar, between 50 and 65% SiO 2, to that of Ladakh [48].

5.3. Scarcity of high-180 volcanics? The brief review of section 3.1 shows 8180 values near 129~ (but not much higher) to be reasonably common in plutonic granitoids. There may be less evidence as yet for high-180 volcanics. Taylor and Turi [49] and Magaritz et al. [50] have found trachytes and rhyolites with up to 14 and sometimes 17ff~ 8~80. On the other hand, the voluminous Taupo rhyolites and ignimbrites, which have traditionally been ascribed to crustal anatexis, range from 7 to 9%o even when somewhat contaminated [15]. If the Sr isotope ratios and chemistry were any guide, the rhyolites of the still larger Sierra Madre province of Mexico [34,51] may turn out to have similar, apparently oceanic, oxygen isotope compositions. It would therefore be of interest to search for further unaltered high180 volcanics. Winkler [52] and Harris et al. [53] have suggested magmas of granitic composition to crystallise mostly below the surface, due to their presumed anatectic origin and consequent nearsaturation with H20. This remarkably general argument, although not uncontested [54] could receive new support if high-180 vo!canics--of anatectic origin--were in fact rare, and'if the most voluminous extrusive rhyolitic magmas were com-

285

monly low in 180, and perhaps n o t of anatectic origin. Volcanic equivalents of the Himalayan leucogranites may have been rare or absent.

Acknowledgements We are indebted to Sandra Healy for experimental assistance and to Shirley Stuart for effective typing. Thorough anonymous reviews proved most helpful and G.A. Challis and A.J. Tulloch provided further useful comment. C.J. Adams and Jin Chengwei are thanked for permission to quote results obtained jointly.

References 1 H.H. Hayden, The geology of the provinces of Tsang and U in Tibet, Geol. Surv. India Mem. 36/2, 122-201, 1907. 2 A. Gansser, Geology of the Himalayas, 289 pp., Wiley-Interscience, London, 1964. 3 A. Gansser, The timing and significance of orogenic events in the Himalaya, in: Geology, Geological History and Origin of Qinghai-Xizang Plateau, pp. 23-28, Science Press, Beijing, 1981. 4 P. LeFort, Himalayas: the collided range, Am. J. Sci. 275-A, 1-44, 1975. 5 Jin Chengwei and Zhou Yunsheng, Igneous rock belts in the Himalayas and the Gangdese arc and their genetic model, Scientia Geol. Sinica 4, 297-312, 1978. 6 J. Hamet and C.J. Ali~gre, Rb-Sr systematics in granite from central Nepal (Manaslu), Geology 4, 470-472, 1976. 7 P. Vidal, Rb-Sr systematics in granite from central Nepal (Manaslu): significance of the Oligocene age and high 87Sr/S6Sr ratio in Himalayan orogeny--Comment to Hamet and All~gre (1976), Geology 6, 196, 1978. 8 V. Dietrich and A. Gansser, Leucogranites of the Bhutan Himalaya, Schweiz. Mineral. Petrogr. Mitt. 61, 177-201, 1981. 9 K. Honegger, V. Dietrich, W. Frank, A. Gansser, M. Th/Sni and V. Trommsdorff, Magmatism and metamorphism in the Ladakh Himalayas, Earth Planet. Sci. Lett. 60, 253-292, 1982. 10 C.J. All~gre and D. Ben-Othman, Nd-Sr isotopic relationship in granitoid rocks and continental crust development: a chemical approach to orogenesis, Nature 286, 335-346, 1980. 11 R.N. Clayton and T.K. Mayeda, The use of bromine pentafluoride in the extraction of oxygen from oxides and silicates for isotopic analysis, Geochim. Cosmochim. Acta 27, 43-52, 1963. 12 P. LeFort, Manaslu leucogranite; a collision signature of the Himalaya. A model for its genesis and emplacement, J. Geophys. Res. 86, 10545-10568, 1981.

13 H.P. Taylor, Jr., Oxygen and hydrogen isotope relationships in hydrothermal mineral deposits, in: Geochemistry of Hydrothermal Ore Deposits, H.L. Barnes, ed., 2nd ed., pp. 236-277, John Wiley and Sons, New Yor, N.Y., 1979. 14 N.C. Ghose and N.K. Singh, Experimental study of granitic rocks of Darjeeling and its application to the origin of Himalayan granites, Tectonophysics 43, 23-40, 1977. 15 P. Blattner and F. Reid, The origin of lavas and ignimbrites of the Taupo Volcanic Zone, New Zealand, in the light of oxygen isotope data, Geochim. Cosmochim. Acta 46, 1417-1429, 1982. 16 H.P. Taylor, Jr., The effects of assimilation of country rocks by magmas on 180/160 and S7Sr/S6Sr systematics in igneous rocks, Earth Planet. Sci. Lett. 47, 243-254, 1980. 17 H.P. Taylor, Jr., The oxygen isotope geochemistry of igneous rocks, Contrib. Mineral. Petrol. 19, 1-71, 1968. 18 J.R. O'Neil and B.W. Chappell, Oxygen and hydrogen isotope relations in the Berridale batholith, J. Geol. Soc. London 133, 559-571, 1977. 19 U. Masi, J.R. O'Neil and R.W. Kistler, Stable isotope systematics in Mesozoic granites of central and northern California and southwestern Oregon, Contrib. Mineral. Petrol. 76, 116-126, 1981. 20 D.E. Lee, I. Friedman and J.D. Gleason, Map showing the oxygen isotope composition of granitoid rocks of the Basin-Range province, U.S. Geol. Surv. Misc. Field Stud., Map MF 1305, 1981. 21 R.S. Harmon and A.N. Halliday, Oxygen and strontium isotope relationships in the British late Caledonian granites, Nature 283, 21-25, 1980. 22 Y. Matsuhisa, A. Sasaki, K. Shibata and S. Ishihara, Source diversity of the Japanese granitoids: an O, S, and Sr isotopic approach, in: Abstracts, 5th International Conference on Isotope Geology, pp. 239-240, Nikko, 1982. 23 Y.-N. Shieh and H.P. Taylor, Jr., Oxygen and hydrogen isotope studies of contact metamorphism in the Santa Rosa range, Nevada, and other areas, Contrib. Mineral. Petrol. 20, 306-356, 1969. 24 H. Honma and H. Sakai, Oxygen isotope study of metamorphic and granitic rocks of the Yanai District in the Ryoke belt, Japan, Contrib. Mineral. Petrol. 52, 107-120, 1975. 25 A. Michard-Vitrac, F. Albar&ie, C. Dupuis and H.P. Taylor, Jr., The genesis of Variscan (Hercynian) phitonic rocks; inferences from Sr, Pb, and O studies on the Maladeta igneous complex, central Pyrenees (Spain), Contrib. Mineral. Petrol. 72, 57-72, 1980. 26 D.E. Lee, I. Friedman and J.D. Gleason, The oxygen isotope composition of granitoid and sedimentary rocks of the southern Snake Range, Nevada, Contrib. Mineral. Petrol. 79, 150-158, 1982. 27 F.J. Longstaffe, T.E. Smith and K. Muehlenbachs, Oxygen isotope evidence for the genesis of Upper Paleozoic granitoids from southwestern Nova Scotia, Can. J. Earth Sci. 17, 132-141, 1980. 28 S.M.F. Sheppard, The Cornubian batholith, SW England: D / H and 1s O/16 0 studies of kaolinite and other alteration minerals, J. Geol. Soc. London 133, 573-591, 1977.

286 29 H.G.F. Winkler and H. von Platen, Experimentelle (3esteinsmetamorphose, III. Anatektische Ultrametamorphose kalkhaltiger Tone, Geochim. Cosmochim. Acta 18, 294-316, 1960. 30 A. Gansser, The significance of the Himalayan suture zone, Tectonophysics 62, 37-52, 1980. 31 W. Frank, A. Gansser and V. Trommsdorff, Geological observations in the Ladakh area, Schweiz. Mineral. Petrogr. Mitt. 57, 89-113, 1977. 32 Y. Matsuhisa, Oxygen isotopic compositions of volcanic rocks from the East Japan island arcs and their bearing on petrogenesis, J. Volcanol. (3eotherm. Res. 5, 271-296, 1979. 33 K. Muehlenbachs and G. Byerly, 180 enrichement of silicic magmas caused by crystal fractionation at the (3alapagos spreading center, Contrib. Mineral. Petrol. 79, 76-79, 1982. 34 M.A. Lanphere, K.L. Cameron and M. Cameron, Sr isotope geochemistry of voluminous rhyolitic ignimbrites and related rocks, western Mexico, Nature 286, 594-596, 1980. 35 A. Ewart find J.J. Stipp, Petrogenesis of the volcanic rocks of the North Island New Zealand as indicated by a study of S'lSr/S6Sr ratios and Sr, Rb, K, U, and Th abundances, Geochim. Cosmochim. Acta 32, 699-736, 1968. 36 S.A. Morse, Strontium isotope fractionation in the Kiglapait intrusion, Science 220, 193-195, 1983. 37 Ph. Vidal. A Cocherie and P. LeFort, Geochemical investigations of the origin of the Manaslu leucogranite, Nepal, (3eochim. Cosmochim. Acta 46, 2279-2292, 1982. 38 H.P. Taylor, Jr., Water/rock interactions and the origin of H20 in granitic batholiths, J. Geol. Soc. London 133, 509-558, 1977. 39 P. Blattner, Combined stable isotope trajectories for water" rock interaction, in: Proceedings, 3rd N.Z. Geothermal Workshop, pp. 75-80, University of Auckland, 1981). 40 (3. Ferrara, B. Lombardo and S. Tonarini, R b / S r geochronology of granites and gneisses from the Mount Everest Region, Nepal Himalaya, Geol. Rundsch. 72, 119-136, 1983. 41 K.R. Mehnert, Petrographie und Abfolge der Granitisation in Schwarzwald, III. Partielle Anatexis, Neues Jahrb. Mineral. Abh. 98, 208-249, 1962.

42 K.R. Mehnert, Petrographie und Abfolge der Granitisation in Schwarzwald, IV. VollstAndige Anatexis, Neues Jahrb. Mineral. Abh. 99, 161-199, 1963. 43 K. Notsu, Strontium isotope composition in volcanic rocks from Japan, in: IAVCEI Symposium, Arc Volcanism, Tokyo and Hakone, p. 265, 1981. 44 Unpublished data of P. Blattner (180/160) and C.J. Adams and P. Blattner (STSr/S6Sr); wide range of samples, 1975-1982. 45 C.A. Fleming, The Mesozoic of New Zealand: Chapters in the history of the Circum-Pacific mobile belt, Q. J. Geol. Soc. London 125, 125-170, 1970. 46 P. Blattner, "Halogen geothermometry" and case for post-Devonian granolite in Fiordland, N.Z. Bull. Mineral. 104, 811-816, 1981. 47 D. DePaolo, Trace element and isotopic effects of combined wallrock assimilation and fractional crystallisation, Earth Planet. Sci. Lett. 53, 189-202, 1981. 48 Jin Chengwei, Academy Sinica, personal communication. 49 H.P. Taylor, Jr. and B. Tuff, HighDSO igneous rocks from the Tuscan magmatic province, Central Italy, Contrib. Mineral. Petrol. 55, 33-54, 1976. 50 M. Magaritz, D.J. Whitford and D.E. James, Oxygen isotopes and the origin of high S7Sr/a6Sr andesites, Earth Planet. Sci. Lett. 40, 220-230, 1978. 51 M. Cameron, W.C. Bagby and K.L. Cameron, Petrogenesis of voluminous mid-Tertiary ignimbrites of the Sierra Madre Occidental, Chihuahua, Mexico, Contrib. Mineral. Petrol. 74, 271-284, 1980. 52 H.(3.F. Winkler, Viel Basalt und wenig (3abbro-wenig Rhyoht und viel Granit, Contrib. Mineral. Petrol. 8, 222-231, 1962. 53 P.(3. Harris, W.Q. Kennedy and C.M. Scarfe, Volcanism versus plutonism--the effect of chemical composition, in: Mechanism of Igneous Intrusion, (3. Newall and N. Rast, eds., pp. 187-200, Gallery Press, Liverpool, 1970. 54 A. Ewart, W. Hildreth and I.S.E. Carmichael, Quaternary acid magma in New Zealand, Contffb. Mineral. Petrol. 51, 1-27, 1975.