The Dir-Utror metavolcanic sequence, Kohistan arc terrane, northern Pakistan

Journal of Asian Earth Sciences 17 (1999) 459±475

The Dir-Utror metavolcanic sequence, Kohistan arc terrane, northern Pakistan Mohammad Tahir Shah a, b, John W. Shervais a,* a

Department of Geological Sciences, University of South Carolina, Columbia, SC 29208, USA b National Centre of Excellence in Geology, University of Peshawar, Peshawar, Pakistan Received 10 February 1999; accepted 15 February 1999

Abstract The Dir-Utror volcanic series forms a NE±SW trending belt within the northwestern portion of the Kohistan island arc terrane in the western Himalayas of northern Pakistan. The Kohistan arc terrane comprises a diverse suite of volcanic, plutonic, and subordinate sedimentary rocks of late Mesozoic to Tertiary age, developed prior to and after suturing of the Indo-Pakistan and Asiatic continental blocks. The Dir-Utror volcanic series near Dir is dominated by basaltic-andesite and andesite, with subordinate basalt, high-MgO basalt, dacite, and rhyolite. Porphyritic textures are dominant, with less common aphyric and seriate textures. Plagioclase is the dominant phenocryst in ma®c to intermediate rocks, K-feldspar and quartz phenocrysts predominate in the dacites and rhyolites. Chlorite, epidote, albite, and actinolite are the most common metamorphic phases; blue-green amphibole, andesine, muscovite, biotite, kaolinite, sericite, carbonate, and opaques are widespread but less abundant. Phase assemblages and chemistry suggest predominant greenschist facies metamorphism with epidote-amphibolite facies conditions attained locally. Whole rock major element compositions de®ne a calc-alkaline trend: CaO, FeO, MgO, TiO2, Al2O3, V, Cr, Ni, and Sc all decrease with increasing silica, whereas alkalis, Rb, Ba, and Y increase. MORB-normalized trace element concentrations show enrichment of the low-®eld strength incompatible elements (Ce, La, Ba, Rb, K) and deep negative Nb, P, and Ti anomaliesÐ patterns typical of subduction related magmas. Ma®c volcanic rocks plot in ®elds for calc-alkaline volcanics on trace element discrimination diagrams, showing that pre-existing oceanic crust is not preserved here. All rocks are LREE-enriched, with La=16±112  chondrite, La/Lu=2.6±9.8  chondrite, and Eu/Eu=0.5±0.9. Dacites and rhyolites have the lowest La/Lu and Eu/Eu ratios, re¯ecting the dominant role of plagioclase fractionation in their formation. Some andesites have La/Lu ratios which are too high to result from fractionation of the more ma®c lavas; chondrite-normalized REE patterns for these andesites cross those of the basaltic andesites, indicating that these lavas cannot be related to a common parent. The high proportion of ma®c lavas rules out older continental crust as the main source of the volcanic rocks. The scarcity of more evolved felsic volcanics (dacite, rhyolite) can be explained by the nature of the underlying crust, which consists of accreted intra-oceanic arc volcanic and plutonic rocks, and is ma®c relative to normal continental margins. Andesites with high La, La/ Lu, K2O, and Rb may be crustal melts; we suggest that garnet-rich high-pressure granulites similar to those exposed in the Jijal complex may be restites formed during partial melting of the crust. # 1999 Elsevier Science Ltd. All rights reserved.

1. Introduction The Kohistan arc terrane of the western TransHimalaya exposes a remarkable cross-section through an island arc sequence which developed as a result of the northward subduction of neo-Tethyan oceanic

* Corresponding author.

crust beneath Asia during late Jurassic and early Cretaceous times (Tahirkheli et al., 1979; Searle et al., 1987). The arc terrane is bounded by two major faults: the Northern Suture or Main Karakoram Thrust in the north and the Indus-Tsangpo Suture or Main Mantle Thrust in the south (Fig. 1). These faults separate arc rocks of the Kohistan terrane from continental rocks of Asia to the north and the Indo-Pakistan continent to the south. Both the Main Mantle Thrust and

1367-9120/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 1 3 6 7 - 9 1 2 0 ( 9 9 ) 0 0 0 0 9 - 7

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M. Tahir Shah, J.W. Shervais / Journal of Asian Earth Sciences 17 (1999) 459±475

Fig. 1. Geologic map of northern Pakistan showing the Kohistan arc terrane and adjacent areas. The area studied here is located near Dir in western Kohistan. MKT, Main Karakoram Thrust; MMT, Main Mantle Thrust (Indus-Tsangpo suture). Other abbreviations: P, Peshawar; K, Kalam; AD, Abbottabad; M, Manshera; B, Besham; ST, Swat; Cs, Chilas; NP, Nanga Parbat; R, Rakaposhi; G, Gilgit; Y, Yasin.

Main Karakoram Thrust are characterized by discontinuous outcrops of blueschist and ophiolite in serpentinite or shale-matrix melange (Tahirkheli et al., 1979; Bard et al., 1980; Bard, 1983; Coward et al., 1982, 1986; Pudsey et al., 1985a; Pudsey, 1986; Hanson, 1989). Recent tectonic models for the development of the western Trans-Himalaya suggest that the KohistanLadakh terrane, which began as an intra-oceanic island arc in the late Jurassic (Dietrich et al., 1983), became a continental Andean-type arc on the southern margin of the Asiatic plate after the closure of a small back arc basin along the Main Karakoram Thrust in the mid Cretaceous (1100 Ma; Tahirkheli et al., 1979; Searle et al., 1987). Continued northward subduction of the Tethyan lithosphere led to development of a ``successor arc'' in the late Cretaceous to late Paleocene, built upon the accreted Mesozoic arc. Initial closure of the Neo-Tethys ocean occurred ca 50±55 Ma, followed by underthrusting of IndoPakistan beneath Asia along the Main Mantle Thrust (Searle, 1983; Searle et al., 1987; Petterson and Windley, 1985; Coward et al., 1986; Rowley, 1996). Our purpose here is to present new petrographic,

mineralogic and geochemical data for the Dir-Utror volcanic series, which represents the ``successor arc'' in northwestern part of the Kohistan arc, and to use these data to infer their origin and relationship to older rocks of the Kohistan terrane. 2. Geologic setting The Kohistan terrane comprises seven major elements (from north to south): the Yasin Group sediments (Tahirkheli and Jan, 1984; Pudsey, 1986), the Cretaceous Chalt volcanic group (Tahirkheli, 1979; Pudsey et al., 1985b; Coward et al., 1982), the Kohistan batholith (Coward et al., 1986; Petterson and Windley, 1985, 1986), the Dir-Utror volcanic series (Tahirkheli, 1979; Majid et al., 1981; Shah, 1991; Shah et al., 1994a; Sullivan et al., 1994), the Chilas layered complex (Hamidullah and Jan, 1986; Khan et al., 1989), the Kamila (southern) amphibolite belt (Jan, 1988; Shah et al., 1992), and the Jijal ma®c-ultrama®c complex (Jan and Howie, 1981). These units represent a complete pro®le through the crust, from relatively shallow supra-crustal levels through high pressure

M. Tahir Shah, J.W. Shervais / Journal of Asian Earth Sciences 17 (1999) 459±475

metamorphic rocks formed at deep crustal levels (Fig. 1). This pro®le is complicated by large-scale folds which deform the entire terrane about ENE-trending axes, as well as older faults and folds (Tahirkheli, 1979; Coward et al., 1982, 1986). The Kohistan terrane evolved in three main stages: (1) as an island arc terrane separated from mainland Asia by a small backarc basin (represented by the Yasin Group sediments) that formed during the late Jurassic to early Cretaceous; this arc is represented by primitive arc tholeiites of the Chalt volcanics, and by deformed bi-modal gabbro-diorite/quartz tonalite plutons (Coward et al., 1982, 1986; Honegger et al., 1982; Dietrich et al., 1983; Petterson and Windley, 1985, 1986); (2) as a continental margin arc, formed during the late Cretaceous to late Paleocene, after suturing to mainland Asia in the mid-Cretaceous; the Dir Group and most of the Kohistan batholith (undeformed granites and granodiorites) formed during this stage; and (3) as the leading edge of the upper plate in a collisional orogen. The Kamila amphibolites, the Chilas complex, and the Jijal complex are all associated with the early Cretaceous ``Stage 1'' arc, although parts of these units may have formed or been modi®ed later. The Dir-Utror volcanic series, found mainly in the west-central part of the Kohistan terrane, forms a ENE-WSW trending belt from the towns of Utror and

461

Kalam (in Swat) through Dir to the border of Afghanistan (Fig. 1). Additional volcanics of the same age are found west of Gilgit (Treloar et al., 1989). The Dir-Utror volcanic series consists of metamorphosed calc-alkaline volcanics and associated metasediments (Tahirkheli, 1979; Majid et al., 1981; Shah, 1991; Shah et al., 1994a; Sullivan et al., 1994) preserved along the axis of the Jaglot syncline (Coward et al., 1982). Treloar et al. (1989) present 39Ar±40-Ar ages of 58±61 Ma (22) for hornblendes separated from ma®c volcanics in the Kalam area and from the area west of Gilgit. These ages are only slightly older than the age of the initial collision of India with Asia ca 55±50 Ma (Rowley, 1996). 3. Field relations Our study area covers about 50 km2 in the vicinity of Dir town in northwestern Pakistan, between latitude 3589 '480 to 35811 '480 and longitude 71848'300 to 7282 ' (Fig. 2). The data reported here are based on ®eldwork carried out by Shah and Shervais during the spring of 1989, and by Shah in the spring and summer of 1990. The area around Dir was mapped at a scale of 1/ 25,000 (Fig. 2) and samples were collected to represent the complete range in rock types and compositions

Fig. 2. Geologic map of the area around Dir, mapped at a scale of 1/25,000. Dir-Utror volcanic series includes felsic metavolcanics, ma®c metavolcanics, pyroclastic breccia, and sheared metavolcanics; all are intruded by later stocks of undeformed diorite, which form the locus of copper mineralization. Metadiorites=Lowari pluton, Meta-arkosic sandstone=Panakot meta-arkose, Epiclastic meta-siltstone=Baraul banda slate.

462

M. Tahir Shah, J.W. Shervais / Journal of Asian Earth Sciences 17 (1999) 459±475

(Fig. 3). Detailed discussions of the ®eldwork and subsequent geochemical investigations may be found in Shah (1991) and Shah et al. (1994a,b). Subsequent to our study Sullivan (1993) conducted a study of the ``root zone'' of the Kohistan batholith, which included work on the Dir Group. Sullivan concluded that metavolcanic rocks of the Dir-Utror volcanic series represents a typical continental margin arc series, based on the apparent abundance of felsic volcanic rocks found on road traverses through the area (Sullivan, 1993; Sullivan et al., 1994). This conclusion is contradicted by our detailed mapping in the Dir area, and by our chemical data, as discussed below. Volcanic and sedimentary rocks in the Swat and Dir areas of western Kohistan were assigned to the Dir group by Tahirkheli (1979, 1982). In the area studied here, these rocks form a fault-bounded sheet between the Lowari quartz metadiorite and low-grade metasediments of the Kalam group (which lies SE of the Dir Group and is not seen on Fig. 2). The Lowari quartz metadiorite, part of the Kohistan batholith, is a relatively homogeneous, moderately foliated, coarsegrained quartz diorite orthogneiss. The Kalam group, of probable Cretaceous age, consists of low-grade metasediments (slates, limestones, and quartzites) which underlie rocks of the Dir group (Tahirkheli, 1979). The Kalam group is intruded by diorites in the south and is in fault contact with slatey shales of Dir Group to the north. Three units are recognized within the Dir group (from the base): the Baraul Banda slate, the Dir-Utror volcanic series, and the Panakot meta-arkose (Tahirkheli 1979, 1982; Shah, 1991). The Baraul

Banda slate is an epiclastic tu€aceous siltstone (called ``tute'' by Fletcher, 1985) up to 4 km thick (Fig. 2). Bedding in this unit strikes approximately E±W and dips steeply to the north. Microfossils in the Baraul Banda slate are early Eocene in age (Tahirkheli, 1979, 1982). The Panakot meta-arkose forms a thin band < 1 km wide which separates volcanic rocks of the Dir-Utror volcanic suite from the Lowari quartz diorite (Fig. 2). The Dir-Utror volcanic series in the area around Dir forms a broad outcrop belt which ranges from <1 km thick in the SW (near Alla) to more than 3 km thick in the NE, near the village of Gumadand (Fig. 2). Ma®c metavolcanic rocks (basalt, basaltic andesite, and andesite) are the most voluminous rock types exposed in the area, comprising over 90% of the outcrop area (including the sheared metavolcanic and pyroclastic breccia units; Fig. 2). They commonly occur as porphyritic ¯ows with elongated plagioclase phenocrysts (<5 mm) oriented subparallel to foliation. The foliation generally strikes NE and dips NW but faulting and folding cause local variations in the trend. Along part of the contact between ma®c metavolcanic rocks and the Baraul Banda slate (interpreted here as a thrust fault), the volcanics are pervasively sheared, creating a strong schistosity. Quartz and epidote veins are common in the sheared metavolcanics, with most veins parallel to schistosity. This deformation, which dies out away from the fault zone, is attributed to southeast-vergent thrusting of the metavolcanics over the Baraul Banda slate. The sheared metavolcanics were subjected to two phases of deformation. The ®rst phase (D1) produced the S1 foliation which generally

Fig. 3. Outline map of geologic units in the Dir area showing location of rock samples studied here. See Fig. 2 for correlation with map units.

M. Tahir Shah, J.W. Shervais / Journal of Asian Earth Sciences 17 (1999) 459±475

strikes NE and dips NW. The D2 deformation is responsible for the crenulation lineation (S2) produced by microfolding of the foliation planes. Felsic metavolcanic rocks (dacite, rhyolite) comprise about 8% of the outcrop area (Fig. 2). They form massive ¯ows and/or ash ¯ow tu€s which generally overlie the ma®c volcanic rocks. The felsic metavolcanic rocks consist of plagioclase, orthoclase, and rare quartz phenocrysts in a ®ne-grained, cryptocrystalline groundmass; aphyric varieties occur as well. Plagioclase and orthoclase phenocrysts (>1.5 mm) are highly fractured and exhibit partial alteration to epidote and clay. Rhyolite dikes (up to 2 m thick) intrude ma®c volcanic rocks near Bikarai village, and may represent feeders to the overlying ¯ows and tu€s. Pyroclastic breccias are widely exposed in the area between Gumadand and Bar Ayagai (Fig. 2). Polymict, matrix-supported breccias are dominant. These breccias consist of subangular to subrounded clasts of basalt and basaltic-andesite, 2±20 cm in diameter (with rare blocks up to 1 m across), set in a ®negrained, maroon-colored matrix of oxidized volcanic debris. The breccia occasionally exhibits sequences in which thin beds (<0.5 m) of ®ne-grained, marooncolored breccia with clasts of <1±4 cm in diameter alternate with thicker (1±2 m) beds of coarse-grained, clast-rich breccia (clasts 2±cm in diameter). The polymict matrix-supported breccias probably represent subaqueous debris ¯ows or lahars. Clast-supported breccias are exposed in the northern portion of the main pyroclastic unit near Bar Ayagai village. These massive, monomict breccias consist of subangular to subrounded ma®c volcanic clasts (up to 2 m across) in a sparse, maroon-colored matrix (clast-to-matrix ratio is about 3:1). The monomict clast-supported breccias probably represent near-vent ¯ow collapse breccias or caldera ®ll. Small stocks and sills of coarse-grained granodiorite and diorite intrude the metavolcanic sequence (Fig. 2). These foliated (pre-kinematic) orthogneisses may represent subvolcanic magma chambers of the felsic metavolcanics. Unlike the Lowari pluton, they are strongly altered to propyllitic assemblages. Disseminated pyrite and chalcopyrite are common, along with supergene enrichment of copper in the form of malachite and azurite. The association of primary copper mineralization with these stocks suggests that they are responsible for Cu mineralization in the surrounding volcanic rocks (Shah, 1991; Shah and Shervais, 1997; Shah et al., 1994b). 4. Mineral chemistry Minerals in the Dir metavolcanics were analyzed with the Cameca SX-50 electron microprobe at the

463

University of South Carolina. The operation conditions were 15 kV accelerating potential and beam current of 25 nA. Natural and synthetic standards were used, with counting times of 20±30 s for each element. Complete mineral data are available from the authors upon request. All of the volcanic rocks have been metamorphosed to low temperature mineral assemblages. These assemblages include chlorite, epidote, actinolite, albite, kaolin, sericite, carbonates, rare hornblende, and opaques in the ma®c metavolcanics; epidote, muscovite, sericite, kaolin, biotite, rare chlorite, calcite and opaques in the felsic metavolcanics. Pseudomorphs after plagioclase and, less commonly ferromagnesian minerals (pyroxene and olivine) are common. Plagioclase is replaced by epidote, carbonates and sericite while pyroxene and olivine are replaced by chlorite and actinolite. Opaque phases include magnetite, hematite, ilmenite and sphene. Actinolites formed early and may be replaced along grain margins by bluish-green hornblende. In some rocks, prismatic green hornblende is the only amphibole; these grains may represent relict hornblende phenocrysts. 4.1. Amphibole Metamorphic amphiboles in the ma®c metavolcanics may be classi®ed variously as actinolites, actinolitic hornblendes, or magnesio-hornblendes. They occur as ®brous to prismatic, colorless to bluish-green grains which are commonly oriented parallel to the foliation. These amphiboles are calcic, with Ca e1.50, Na E1.0, and Si E7.60 atoms per formula unit on the basis of 23 oxygens (Leake, 1978). Fe3+/Fe ratios determined by the method of Laird and Albee (1981) range from 0.18 to 0.46, but most analyses cluster near Fe3+/ Fe=0.30. Ca±Mg±Fe ternary ratios are typical of ma®c metavolcanic rocks, with Mg/Mg+Fe2+=0.44± 0.79. Hornblendes are higher in Al, Na, and Ti, and lower in Si and Mg, than the lower grade actinolites. The increase in the Ti contents from actinolite to hornblende are independent of the bulk rock TiO2 and seems to be due to increase of metamorphic grade (Raase, 1974; Kuniyoshi and Liou, 1976; Hutchison, 1978). Relict igneous phenocrysts of prismatic green Tschermakitic hornblende have the highest Al contents, and may be distinguished from metamorphic blue-green amphiboles by their high Alvi contents (Fig. 4). 4.2. Epidote All the epidotes are iron-rich, with pistaciite contents (Ps=100  Fe/[Fe+Al]) ranging from 24 to 39 mole%. MnO ranges from 0.00 to 0.98 wt%, with negligible TiO2, Na2O, MgO, and K2O. There is no sig-

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M. Tahir Shah, J.W. Shervais / Journal of Asian Earth Sciences 17 (1999) 459±475

crysts and in the groundmass. No primary igneous plagioclase has been found in these rocks. Plagioclase ranges from albite to andesine in composition, with no core-rim zoning (Fig. 5). Plagioclase phenocrysts are commonly peppered with inclusions of secondary epidote, giving them a turbid appearance. In some samples, plagioclases phenocrysts are completely replaced by epidote and other metamorphic phases. As expected, plagioclase in lower grade (greenschist facies) samples are albite+oligoclase, while plagioclase in higher grade (epidote-amphibolite facies) samples are of albite to andesine composition. 4.5. Biotite

Fig. 4. Amphibole compositions plotted on the Alvi ÿ Si diagram of Raase (1974). Amphiboles which plot near pargasite in composition (Alvi>0.6) are thought to represent relict magmatic hornblendes. There is a small compositional gap between greenschist facies actinolites and epidote amphibolite facies edenitic hornblende.

ni®cant change in composition between the cores and rims. Iron-rich epidotes generally indicate low-grade (greenschist facies) metamorphic conditions (e.g. Cooper, 1972). 4.3. Chlorite Chlorite is the most abundant metamorphic mineral in basic to intermediate members of the sequence. They range in composition between ripidolite to brunsvigite, with low or negligible concentrations of elements (Na, Ca, K) which typically occur in interstrati®ed chlorite-smectite (e.g. Ewarts and Schi€man, 1983; Bettison and Schi€man, 1988). Chlorites from di€erent rocks may di€er considerably in composition, while chlorites from individual rock samples show little or no variation. Chlorites in low grade rocks which coexist with actinolite are lower in MgO (<14 wt%) and higher in FeO (>26 wt%) than chlorites in higher grade rocks which coexist with bluegreen hornblende. High grade chlorites are higher in MgO (>17 wt%) and lower in FeO (<24 wt%), consistent with the earlier work on natural mineral assemblages (Cooper, 1972; Kurata and Banno, 1974; Ishizuka, 1985).

Biotite occurs as small ¯akes interspersed in the groundmass of andesites, dacites and rhyolites. Textural features show that these biotites formed at the expense of chlorite, muscovite and iron ore (magnetite). These biotites are the Mg-rich variety, with Mg/[Mg+Fe] ratios of 0.50 to 0.61 and low Al contents. 4.6. Muscovite Muscovite occurs as scattered ¯akes in the groundmass of altered ma®c as well as felsic metavolvcanics. Almost all of the muscovites analyses plot near the Alvi apex in a triangular diagram of Alvi, Fe+Mn, and Mg. Potassium is the dominant interlayer cation, with minor Na (0.04±0.07 atoms per formula unit) and negligible Ca. 5. Whole rock geochemistry Sixty of the least altered samples, representing every major rock unit of the volcanic series, were analyzed for major and trace element composition using a Philips PW-1400 X-ray ¯uorescence spectrometer at the University of South Carolina; the results are presented in Table 1. Samples were prepared for both major and trace element analyses as pressed powder

4.4. Plagioclase Plagioclase is ubiquitous in all the members of the metavolcanic sequence, where it occurs both as pheno-

Fig. 5. Feldspar compositions in Or±Ab±An ternary. Most feldspars are low in both An and Or, and range in composition from pure albite to An42.

M. Tahir Shah, J.W. Shervais / Journal of Asian Earth Sciences 17 (1999) 459±475

465

Table 1 Whole rock geochemical data by X-ray ¯uorescence spectrometery for the Dir-Utror volcanic series near Dir (Swat District), northern Pakistan. Major elements in wt% oxide, trace elements in parts per million (ppm). Totals recalculated to 100% anhydrous, with LOI in wt% High-MgO Basalt DR467 SiO2 TiO2 Al2O3 Fe2O3a MnO MgO CaO Na2O K2O P2O5 LOI ppm Ti Nb Zr Y Sr Rb Ni Cr V Sc Ba

49.13 1.08 15.70 11.26 0.22 13.71 6.12 2.35 0.05 0.39 4.88 6464 11 158 28 704 0 150 362 315 34 9

DR52 48.23 1.18 15.86 10.81 0.21 13.33 6.35 3.04 0.55 0.36 2.84 7074 12 116 23 431 15 77 189 414 28 96

DR360

DR527

48.25 1.35 20.17 10.02 0.16 11.44 3.35 3.78 1.23 0.26 2.12

50.97 1.18 15.87 10.41 0.18 11.11 6.27 3.15 0.59 0.26 4.83

8106 14 134 28 250 34 55 155 300 23 101

7089 6 142 24 404 13 186 294 184 21 85

Low-MgO Basalt b

DR117A DR104 51.53 1.07 15.74 9.25 0.20 9.86 7.15 4.71 0.26 0.23 5.21 6435 4 140 24 322 8 47 106 227 23 24

49.28 0.76 14.07 11.50 0.23 9.31 11.14 3.14 0.34 0.24 7.57 4573 3 74 14 321 5 59 138 210 25 61

DR383

DR70

51.76 0.70 18.25 10.26 0.22 8.27 4.58 4.82 0.92 0.23 4.28

51.47 1.18 15.50 11.46 0.22 6.39 10.34 2.55 0.56 0.33 5.26

4182 0 107 20 331 28 30 48 263 26 164

7059 13 147 33 356 20 40 61 248 25 73

DR311a DR328b 50.93 1.09 16.33 10.50 0.19 7.91 9.11 2.79 0.69 0.45 3.23 6554 11 122 26 485 16 69 191 382 31 90

49.29 0.90 19.74 9.65 0.17 6.96 7.04 4.18 1.81 0.25 2.12 5391 7 99 26 297 63 29 25 216 21 150

DR83b

DR88b

48.06 0.69 17.31 9.93 0.24 7.98 12.48 2.39 0.72 0.21 8.31

48.60 0.72 16.94 10.44 0.23 7.86 11.75 2.59 0.67 0.20 5.79

4124 23 70 20 359 20 125 381 374 32 90

4338 7 68 19 347 15 122 383 394 35 73

DR219

DR398a

DR508

DR459b

54.96 0.75 18.38 9.73 0.13 5.80 6.35 3.28 0.22 0.21 4.28

55.52 0.87 20.16 7.05 0.18 3.85 4.50 5.69 1.99 0.23 2.13

55.65 0.73 15.63 9.53 0.12 5.37 8.33 2.89 1.44 0.25 3.78

55.83 0.71 21.21 7.53 0.19 2.21 6.08 4.34 1.73 0.24 2.70

Basaltic andesite DR131

DR373

DR400

DR436a

DR321

DR63a

DR455a

DR493

52.15 SiO2 1.06 TiO2 16.08 Al2O3 Fe2O3a 10.11 MnO0.24 0.24 MgO 7.65 CaO 8.10 3.25 Na2O K2O 1.02 0.33 P2O5 LOI 5.43 ppm Ti 6379 Nb 4 Zr 143 Y 25 Sr 356 Rb 25 Ni 37 Cr 57 V 294 Sc 26 Ba 169

52.35 0.85 22.83 7.43 0.16 7.31 5.47 2.11 1.15 0.27 3.58

52.89 0.96 19.05 9.74 0.17 6.26 5.71 3.41 1.62 0.19 3.23

53.11 1.06 18.99 8.81 0.17 7.71 6.15 2.60 1.19 0.22 3.05

53.31 0.68 20.09 8.21 0.19 5.99 5.95 4.62 0.66 0.31 3.86

53.60 1.22 15.71 11.05 0.19 7.72 7.97 1.72 0.56 0.26 3.23

53.72 1.03 16.87 9.33 0.10 9.31 5.74 3.23 0.23 0.35 4.10

53.76 0.86 16.38 8.66 0.17 7.66 7.01 3.73 1.56 0.26 3.78

5097 13 148 22 175 42 15 15 248 16 124

5739 5 96 25 288 40 40 57 272 21 472

6344 0 135 27 301 21 7 19 209 18 191

4105 8 128 29 884 16 8 14 153 15 94

7287 8 145 29 389 17 29 38 247 21 51

6184 2 158 22 460 7 54 65 220 21 45

DR431b DR350a 54.61 0.87 19.19 9.10 0.15 8.22 2.99 3.66 1.03 0.16 3.45

54.91 0.78 17.13 9.00 0.31 2.27 9.62 3.29 2.65 0.21 5.13

5153 0 115 19 528 56 16 35 274 26 101

5235 0 90 21 486 32 63 57 247 24 190

4681 0 83 19 411 77 14 37 326 23 249

4524 6 96 28 164 8 48 105 358 19 4

5236 3 153 29 335 39 5 17 168 16 296

4348 12 94 20 361 36 20 30 211 22 341

Andesite

SiO2 TiO2 Al2O3 Fe2O3a MnO MgO CaO Na2O K2O P2O5 LOI ppm Ti Nb Zr Y Sr Rb Ni Cr V Sc Ba

a

DR591

DR197

DR495

DR480

DR10

DR512

DR517

DR263a

DR500

DR309

DR504a

DR11

DR6

57.68 0.52 17.63 7.63 0.17 4.27 6.69 3.21 1.78 0.41 3.78

58.23 0.80 15.40 6.99 0.28 3.59 6.45 4.29 3.70 0.27 2.89

58.69 0.73 17.16 7.71 0.10 3.96 4.72 4.45 2.21 0.26 2.94

59.05 0.90 18.20 7.04 0.30 4.44 2.45 4.71 2.67 0.25 4.97

60.31 0.88 18.23 5.71 0.15 1.27 4.30 4.61 4.17 0.38 2.86

60.58 0.77 18.13 6.87 0.09 3.83 4.55 1.27 3.75 0.17 2.88

60.67 0.77 17.66 7.06 0.10 4.02 3.83 1.52 4.21 0.16 3.69

61.16 0.70 16.81 5.93 0.12 2.53 6.61 3.77 2.14 0.23 2.59

61.97 0.54 14.80 4.89 0.20 2.62 12.63 1.40 0.76 0.18 2.46

59.19 0.68 18.25 6.66 0.09 3.23 6.00 1.34 4.38 0.18 3.28

60.31 0.71 16.92 6.95 0.10 3.92 5.51 1.73 3.68 0.17 3.32

58.24 0.97 14.21 9.09 0.15 2.72 7.84 6.34 0.22 0.21 3.80

60.68 1.18 15.60 8.14 0.12 2.77 2.68 8.39 0.11 0.33 1.86

3145 2 111 20 680 41 14 22 157 14 206

4816 11 150 25 251 85 33 74 155 16 468

4403 6 123 21 419 79 14 25 148 16 283

5397 0 148 30 158 37 11 11 169 15 355

5254 10 201 38 392 100 0 8 65 14 477

4595 9 159 30 182 153 50 58 137 15 229

4620 13 165 24 167 170 53 64 137 13 310

4169 12 164 21 479 52 24 33 162 18 290

3229 9 157 18 151 23 26 90 100 24 85

4080 11 164 23 313 141 38 52 128 13 507

4273 5 159 27 189 121 54 59 134 16 341

5839 4 121 27 263 11 44 66 136 22 8

7054 5 165 43 52 0 10 20 151 25 6

(continued on next page)

4273 13 108 33 609 28 5 14 175 15 298

DR101b

DR313

DR389

DR133

55.20 0.54 13.72 8.48 0.17 5.41 13.21 2.79 0.29 0.17 6.23

54.74 1.25 18.81 9.93 0.16 8.14 3.47 2.71 0.56 0.23 6.06

53.38 0.70 20.50 8.13 0.57 3.83 7.12 2.73 3.11 0.34 6.06

52.10 1.15 18.07 9.25 0.57 5.11 5.75 1.93 5.76 0.30 4.56

3208 6 55 13 709 8 24 51 237 23 3

7494 7 232 32 630 18 65 93 259 23 62

4175 11 137 30 403 58 17 23 180 14 329

6883 11 103 18 313 81 19 22 136 15 597

466

M. Tahir Shah, J.W. Shervais / Journal of Asian Earth Sciences 17 (1999) 459±475 Dacites

SiO2 TiO2 Al2O3 Fe2O3a MnO MgO CaO Na2O K2O P2O5 LOI ppm Ti Nb Zr Y Sr Rb Ni Cr V Sc Ba a b

DR499

DR216

64.09 0.54 12.51 4.66 0.15 2.28 9.33 4.26 1.97 0.20 2.45

65.92 0.73 16.21 5.84 0.19 1.23 3.26 5.30 1.19 0.13 2.36

3208 12 112 24 300 49 27 73 104 15 357

4404 6 88 24 186 46 13 32 87 17 152

DR47 68.15 0.44 15.61 5.75 0.15 2.61 0.25 6.63 0.29 0.13 2.16 2611 7 167 28 56 11 16 14 90 8 10

Rhyoites a

DR17

DR22

DR50A

DR286

DR34A

DR192

68.90 0.25 16.21 2.90 0.17 0.76 2.83 4.83 2.99 0.17 2.00

69.08 0.24 16.11 2.99 0.21 0.63 3.76 4.06 2.76 0.18 2.14

69.19 0.75 15.08 5.18 0.11 2.90 1.77 3.72 1.00 0.29 2.78

69.60 0.53 16.32 2.41 0.05 0.44 1.01 6.35 3.16 0.11 2.04

66.81 0.63 14.77 5.25 0.22 1.28 1.10 1.30 8.49 0.16 2.65

68.79 0.81 15.71 2.65 0.15 0.34 0.61 3.98 6.72 0.23 1.75

1485 7 115 24 153 75 0 6 5 4 327

1426 6 114 19 393 77 1 5 2 7 415

4509 10 180 34 158 16 4 3 98 12 149

3200 14 260 43 119 71 3 6 30 11 310

3796 12 193 36 71 56 4 5 78 14 467

4884 12 204 39 56 78 1 5 20 9 599

a

DR4

70.17 0.67 14.76 3.95 0.08 0.24 0.86 6.28 2.84 0.15 0.58 4043 8 200 43 95 77 3 10 37 14 245

a

DR78

DR250

70.98 0.22 15.03 2.55 0.06 1.58 0.32 7.73 1.35 0.18 1.65

70.99 0.42 15.16 2.99 0.04 0.20 0.73 7.95 1.40 0.11 0.38

1328 7 146 23 47 7 9 11 11 5 5

2530 11 214 62 68 14 5 6 23 14 65

DR214A DR218A 71.69 0.37 14.86 3.26 0.15 1.39 1.83 4.65 1.70 0.10 1.78 2203 2 223 40 57 87 6 10 105 8 245

72.13 0.34 13.62 3.49 0.13 0.99 2.59 4.73 1.80 0.16 1.78 2064 2 214 21 126 73 13 18 98 8 194

DR174

DR29a

DR230

72.66 0.29 14.44 1.88 0.07 0.36 2.60 6.06 1.58 0.06 1.73

73.34 0.35 14.17 1.74 0.08 0.43 1.13 6.63 2.07 0.08 1.05

73.55 0.41 13.18 2.26 0.08 0.38 3.03 4.67 2.33 0.11 1.34

1724 10 190 34 104 46 5 5 11 11 45

2071 0 232 49 92 41 3 5 16 11 258

2443 7 173 35 110 53 4 6 35 11 97

REE analysis. Breccia unit.

from 10.4 wt% to 18.3 wt%, making direct comparisons of the analyses impossible. As a result, the whole rock chemical data have been recalculated to 100% on volatile-free basis; the volatile-free analyses and LOI are shown in Table 1. Fourteen representative samples were analyzed for rare earth elements (Table 2). Samples for REE analyses were prepared using an ion exchange technique following the method of Crock and Lichte (1982) and analyzed by ICP/OES (inductively coupled plasma/ optical emission spectroscopy) in the geochemistry laboratory of Eastern Washington University. In this method, the powdered sample is fused with lithium metaborate, dissolved into distilled water, and evaporated to dryness. Separation and reconcentration of the REE were made by nitric acid gradient cation exchange and hydrochloric acid anion exchange. Precision and accuracy of 2±5% relative were observed.

briquettes using a 2% polyvinyl alcohol solution as the binder; this binder is transparent to X-rays and results in an e€ective dilution factor of zero (Holland and Brindle, 1966). Calibration lines were constructed using U.S.G.S. and other international rock standards which match the range of compositions found in the samples. Recommended concentrations for the standards were taken from the compilation of Potts et al. (1992). Matrix corrections were carried out within the Philips X41 software package, which uses fundamental parameters approach (Rousseau, 1989) to calculate theoretical alpha coecients for the range of standards. Replicate analyses of selected standards as unknowns suggest percent relative errors of around 1% for silica, 2±4% for less abundant major elements, 6±7% for minor elements with concentrations <0.5 wt%, and 11±6% for the trace elements. Loss on ignition (LOI) was performed in duplicate by heating 12 g sample at 9508C for more than 4 h. LOI varies

Table 2 Rare earth element analysis by ICP-MS for selected whole rock samples. Rock types: HMB, high-Mg basalt; LMB, low-Mg basalt; Bas-And, basaltic andesite Sample No. DR104 DR311 DR436 DR63 DR455 DR350 DR398 DR591 DR504 DR263 DR192 DR4 DR250 DR29 Rock Type HMB LMB Bas-And Bas-And Bas-And Bas-And Bas-And Andesite Andesite Andesite Dacite Rhyolite Rhyolite Rhyolite La Ce Nd Sm Eu Gd Dy Er Yb Lu

5.15 9.7 6.35 0.4 0.385 2.00 2.045 1.05 1.13 0.165

9.50 10.45 21.4 29.1 8.6 7.45 1.75 2.9 0.570 0.670 2.35 2.85 2.095 3.99 1.05 2.15 1.10 2.44 0.160 0.325

10.30 25.8 14.35 4.4 0.910 3.95 4.945 2.90 2.85 0.405

11.35 28.05 15.6 4.45 0.900 3.00 3.445 1.40 1.73 0.255

11.60 25.85 11.3 2.75 0.695 2.45 3.055 1.55 1.72 0.235

11.20 25.7 11.85 2.7 0.630 2.55 3.545 2.10 1.88 0.220

16.6 31.3 13.35 2.5 0.665 2.35 2.37 1.05 1.37 0.19

36.9 77.75 29.2 6.65 1.04 5.9 4.915 2.35 2.775 0.39

12.0 27.1 12.35 3.15 0.675 2.85 3.37 1.75 1.925 0.255

17.4 14.6 43.1 37.3 20.2 20.45 6.15 6.4 0.925 1.175 5.15 5.00 6.315 5.87 3.8 3.4 4.34 3.64 0.665 0.52

17.85 40.9 24.45 7.75 1.53 6.6 8.235 4.4 4.74 0.71

20.25 47.5 21.4 6.35 0.98 5.4 5.86 3.05 3.5 0.505

M. Tahir Shah, J.W. Shervais / Journal of Asian Earth Sciences 17 (1999) 459±475

467

Fig. 6. Major element and trace element compositions of Dir-Utror metavolcanic rocks plotted on silica-variation (Harker) diagrams. The DirUtror metavolcanics display typical calc-alkaline trends, with Mg, Fe, Ca, Al, Ti, Sr, Cr, V, and Sc decreasing as silica increases. Alkalis, Zr, and Y increase with increasing silica. Most of the observed scatter can be attributed to element mobility during low-grade metamorphism. Small symbols show fractionation paths of high-MgO basalt, low-MgO basalt, and a primitive basaltic andesite, calculated using the program ``Comagmat'' by Ariskin et al. (1993).

468

M. Tahir Shah, J.W. Shervais / Journal of Asian Earth Sciences 17 (1999) 459±475

5.1. Major elements Volcanic rocks of the Dir-Utror volcanic series include high-MgO basalts (MgO > 9.0 wt%), lowMgO basalts (5±9 wt% MgO), basaltic-andesites, andesites, dacites, and rhyolites. Clasts in the pyroclastic breccia unit are all basalt or basaltic-andesite, as are almost all of the sheared volcanics. Calculated volumes of each rock type based on outcrop areas and the distribution of analyses within each map unit suggest the following proportions: high-MgO basalts=12% by volume, low-MgO basalts=23% by volume, basaltic-andesites=34% by volume, andesites=37% by volume, dacites=4% by volume, and rhyolites=4% by volume. The distribution of compositions reported here is similar to that observed in mature island arcs such as southwest Honshu (Ewart, 1982) or the Aleutian arc (Marsh, 1982) and some continental margin arcs built on older arc-derived crust (e.g. the southern volcanic zone of Chile; HickeyVargas et al., 1989; Davidson et al., 1988; the Cascades, McBirney and White, 1982). The major element data for the Dir-Utror volcanics are plotted on Harker variation diagrams in Fig. 6. The data approximate a liquid line of descent from basalt to rhyolite, although most oxides show some scatter. MgO, Fe2O3, TiO2, CaO, Al2O3, and P2O5 all show negative correlation with SiO2, while the alkalis Na2O and K2O both show positive correlation. The scattering of Na2O, K2O, and CaO can be attributed to the gain or loss of these oxides during low-grade regional metamorphism or exchange with seawater. Some high-MgO basalts may re¯ect Mg-enrichment during metamorphism, but others have high Cr and Ni concentrations as well, suggesting that the high MgO is primary. The lack of iron and TiO2 enrichment is

Fig. 8. Chondrite-normalized REE diagrams for selected volcanic samples: (a) Group 1=DR104, DR63, DR192, DR4, DR29, DR250; (b) Group 2=DR350, DR398, DR436, DR455, DR263; (c) Group 3=DR311, DR591; and Group 4=DR504. Samples are grouped according to Ce/Yb ratios and Eu/Eu anomalies (see text).

characteristic of calc-alkaline rocks. Most samples plot within the calc-alkaline ®eld on an AFM plot, and the whole series exhibits a typical calc-alkaline trend (Fig. 7). 5.2. Trace elements

Fig. 7. Alkali±FeO±MgO ternary diagram with calc-alkaline/tholeiite dividing line of Irvine and Barager (1971). Volcanics of the DirUtror series are dominantly calc-alkaline in character.

The trends shown by most trace elements on silica variation diagrams are consistent with the combined e€ects of fractional crystallization and hydrothermal mobilization (Fig. 6). There is a strong correlation between silica and elements which are immobile during hydrothermal metamorphism: the compatible elements Cr, Ni, V, and Sc all decrease with increasing SiO2 while Zr and Y increase (Fig. 8). The incompatible elements Rb, Ba, and Sr scatter against SiO2, re¯ecting the mobility of these elements during low-temperature metamorphism and (for Sr) the variable distribution of

M. Tahir Shah, J.W. Shervais / Journal of Asian Earth Sciences 17 (1999) 459±475

469

plagioclase phenocrysts. The overall trend for Sr shows depletion from basalt to rhyolite in this sequence. 5.3. Rare earth elements Chondrite-normalized REE patterns show that all members of the Dir-Utror volcanic series are enriched in light rare earth elements (LREE) compared to the heavy rare earth elements (HREE), and that many have distinct negative Eu anomalies (Fig. 8). Total REE concentrations increase from basalt to rhyolite [Fig. 9(a): SiO2 vs Cen], but LREE-enrichments do not correlate with silica content [Fig. 9(b): SiO2 vs Cen/

Fig. 10. REE ratio diagrams: (a) Ce/chondrite vs Ce/Yb ratio, showing wide range in Ce/Yb ratios that does not correlate with Ce concentration; (b) Ce/chondrite vs Eu/Eu, where Eu is interpolated Eu concentration, showing good negative correlation between the size of the Eu anomaly and Ce content for all samples except Group 4 (DR504); (c) Ce/Yb ratio vs Eu/Eu, showing good positive correlation for all samples except Group 4 (DR504) and two Group 1 samples (DR192, DR29). Symbols same as Fig. 9.

Fig. 9. REE concentrations and ratios as a function of silica content: (a) SiO2 vs Ce/chondrite, showing strong positive correlation; (b) SiO2 vs chondrite-normalized Ce/Yb ratio, showing wide range in Ce/Yb ratios that does not correlate with SiO2; (c) SiO2 vs Eu/Eu, where Eu is interpolated Eu concentration, showing moderately good negative correlation between the size of the Eu anomaly and silica content for all samples except Group 4 (DR504). Symbols: W, Group 1; R, Group 2; Q, Group 3; and q, Group 4.

Ybn] or with REE concentration [Fig. 10(a): Cen vs Cen/Ybn]. Negative Eu anomalies increase with increasing silica and REE concentration [Fig. 9(c): SiO2 vs Eu/Eu, and Fig. 10(b): Cen vs Eu/Eu, where Eu is the straight-line interpolation between Sm and Gd]. Eu/Eu also shows a strong positive correlation with LREE enrichment [e.g. [Ce/Yb]n, Fig. 10(c)]. At least four groups of lavas can be distinguished based on their REE systematics. Group 1 volcanics, comprising high-MgO basalt, basaltic andesite, dacite, and rhyolite, are characterized by slight enrichments of LREE (Cen/Ybn < 3.5), and moderate to deep negative Eu anomalies in the more felsic samples (Eu/ Eu=0.7±0.5). Group 2 volcanics, comprising basaltic

470

M. Tahir Shah, J.W. Shervais / Journal of Asian Earth Sciences 17 (1999) 459±475

andesite and andesite, are characterized by higher LREE/HREE ratios (Cen/Ybn=3±4) and small negative Eu anomalies (Eu/Eu=0.68±0.81). Group 3 comprises two samples (low-MgO basalt and an andesite) with high LREE/HREE ratios (Cen/Ybn=5±6) and slight negative Eu anomalies (Eu/Eu=0.84±0.87). These lavas have HREE concentrations that are lower than the basaltic andesites and andesites of groups 1 and 2, which results in crossing patterns when all are plotted together on a single chondrite-normalized REE diagram. These crossing REE-patterns cannot be explained by simple models involving fractional crystallization or magma-mixing. Group 4 consists of a single sample, andesite DR504, with high total REE (Lan=112  chondrite), extreme LREE-enrichment (Cen/Ybn=7.2), and a deep negative Eu anomaly (Eu/ Eu=0.5). The REE systematics of this sample are unique in our sample set, and it cannot be related to the other samples studied here by any simple fractionation or melting mechanisms; however Sullivan (1993) reports a dacite with similar REE characteristics from his area. The chondrite-normalized REE pattern of our sample (with 60% SiO2) crosses the REE patterns

of rhyolites and dacites in Group 1. The single group 4 sample analyzed here is also higher in Rb (121 ppm) than the main trend of other Dir metavolcanic rocks; other andesites with higher than normal Rb (>100 ppm) may be members of this same group. 6. Discussion 6.1. Origin of the basaltic lavas The high proportion of ma®c lavas found in the Dir-Utror volcanic series resembles ma®c lava proportions in mature oceanic arcs and is unlike the distribution of volcanic rock compositions in ``typical'' continental margin arcs (Ewart, 1982; Hildreth and Moorbath, 1988). Continental margin arcs with high proportions of ma®c lava occur where the underlying arc basement comprises accreted oceanic crust or older island arc terranes (e.g. the Southern Volcanic Zone of Chile: Davidson et al., 1988; Hickey et al., 1986; Hickey-Vargas et al., 1989, or the Cascades: McBirney and White, 1982). The lack of a strong geochemical

Fig. 11. Trace element discrimination diagrams after Pearce and Cann (1973) and Pearce (1975): (a) Ti/100 vs Zr vs Y3 ternary; analyses cluster in ®eld ``C''=calc-alkaline volcanics; (b)Ti/100 vs Zr vs Sr/2 ternary; analyses cluster in ®eld ``B''=calc-alkaline volcanics; (c) Ti vs Zr diagram; analyses cluster in ®eld ``CAB''=calc-alkaline volcanics; (d) Ti vs Cr diagram; analyses cluster in ®eld ``VAB''=calc-alkaline and arc tholeiite volcanics. Only basalts and basaltic andesites are plotted.

M. Tahir Shah, J.W. Shervais / Journal of Asian Earth Sciences 17 (1999) 459±475

contrast between ascending arc magmas and the subarc basement results in marginal arcs which resemble oceanic arcs in their overall chemical composition (Ewart, 1982; Ewart et al., 1977; Gill, 1981). Another possibility is that some or all of the basaltic rocks in the Dir-Utror volcanic series represent preexisting oceanic crust upon which the arc was built. We can test this hypothesis using trace element discrimination diagrams designed to distinguish the tectonic setting of ophiolite terranes. Fig. 11 shows a series of trace element discrimination diagrams after Pearce and Cann (1973). In all of these diagrams basaltic rocks of the Dir-Utror volcanic series plot in the ®elds of orogenic arc basalts, not MORB (Fig. 11). The same results are obtained for a MORB-normalized spider plot (Fig. 12), using the normalization factors of Pearce (1983). The order of elements in this plot is based upon their relative mobility in aqueous ¯uids, which should control enrichment processes in subduction-related environments (Pearce, 1983). All samples show the same general features: enriched large ion lithophile element (K, Rb, Ba) concentrations relative to the less mobile elements, enriched LREE/HREE ratios, and negative anomalies for Nb, Ti, and P (Fig. 12). Two groups of basaltic rocks are recognized in the Dir-Utror metavolcanics: high-MgO basalts with Mgs=62±71, and low-MgO basalts with Mgs=52±66. The low-MgO basalts are generally richer in Al2O3 than the high-MgO basalts, but Al-rich plagioclasephyric rocks are found in both groups. Both groups have similar HFSE concentrations (Ti, Zr, Nb). Most high-MgO basalts are characterized by higher Cr, Ni, and V, and by much lower Rb, than the low-MgO basalts, showing that the di€erence in MgO content is

Fig. 12. Spider-diagram normalized using the average MORB values of Pearce (1983). Symbols: R, high-MgO basalt; T, low-MgO basalt; w, basaltic andesites; no symbols, andesites, +, dacites and rhyolites. All samples are enriched in LILE (K, Rb, Ba, Ce) with distinct negative anomalies for Nb, P, and Ti.

471

not a function of low-temperature metamorphism in most cases. None-the-less, we cannot rule out the gain (or loss) of MgO by some samples in response to sea¯oor alteration or low-temperature regional metamorphism. Table 3 summarizes the results of partial melting calculations designed to test relationships between the two basalt groups. The limited REE data for these rocks suggests that the low-MgO and high-MgO basalts cannot be derived from the same source region at constant pressure. Partial melt models show that hypothetical spinel peridotite and garnet peridotite source regions for the low-MgO basalts must have higher Ce/Yb ratios (1.6±2.6) than those for the highMgO basalts (1.0±1.5) if both magmas came from source regions with similar mineralogies. Derivation of both basalt groups from source regions with similar Ce/Yb ratios (=1.5  chondrite) requires that the lowMgO basalt formed by 10±15% partial melting at higher pressures, in equilibrium with garnet, while the high-MgO magmas formed by 20±25% partial melting at lower pressures, within the spinel stability ®eld (Table 3). 6.2. Origin of the evolved lavas Lavas with compositions more evolved than basalt comprise approx. 65% of the mapped volcanic rocks. Possible origins for these rocks include fractional crystallization of more primitive basic magmas, crustal assimilation, magma mixing of primitive basalts with crustal melts, or some combination of these processes. The occurrence of at least four distinct compositional groups based on REE systematics further implies that partial melting of di€erent source regions is required to explain some of these di€erences. All of the evolved lavas have MORB-normalized trace element patterns similar to mature island arc volcanics, with high concentrations of LILE (K, Rb, Ba) relative to HFSE (Nb, Zr, Ti, Y, Yb), negative Ti, P, and Nb anomalies that become deeper with increasing fractionation, and negative Sr anomalies in the andesites, dacites, and rhyolites (Fig. 12). The decrease in Ti, P, and Sr with increasing fractionation re¯ects the calc-alkaline character of the rocks and the fractionation of Ti-magnetite, phosphates, and plagioclase. Rhyolites and dacites of group 1 are characterized by moderate to deep negative Eu anomalies, and are associated with a basaltic andesite with a smaller negative Eu anomaly, and lower overall REE concentrations (Fig. 8). The REE data are consistent with formation of the evolved felsic volcanics from more ma®c parent magmas (basaltic andesites) by plagioclase-dominated crystal fractionation. Basaltic andesites of this group may in turn derive from more primitive high-MgO basalts, but this is dicult to

472

M. Tahir Shah, J.W. Shervais / Journal of Asian Earth Sciences 17 (1999) 459±475

Table 3 Partial melting calculations for presumed ``parent magmas'' based on hypothetical residual mantle and crustal lithologies, and simple modal melting (A) Assumed parent compositions for the melt models, based on representative analyses of each rock type, with REE in each adjusted to yield observed patterns. REE values relative to chondrite, all other elements in ppm ``Starting compositions'' Co ]1: Peridotite to high-MgO Co ]2: Gt Perid to high-MgO Co ]1: Peridotite to low-MgO Co ]2: Gt Perid to low-MgO Co ]3: Gt Gran-15 to low-MgO Co ]4: Gt Gran-25 to low-MgO Co ]5: Gt Amp to DR504 Co ]4: Gt Gran-25 to DR504

Rb 3 3 4 4 4 4 13 16

Sr 105 105 60 60 300 300 50 150

Ba 15 16 17 15 20 20 20 60

Ce 3.1 3 4.5 4.3 6.3 6.9 24.5 23

Sm 2 1.9 2 1.9 3.3 3.4 21 12

Eu 1.7 1.6 1.8 1.7 3.7 3.7 10 12

Yb 2.1 3 1.7 2.7 4.1 5.3 21 13.1

Zr 37 40 37 23 25 25 80 25

Ni 3000 3000 2500 2200 170 170 200 120

Cr 4400 4500 4400 4400 2300 2300 1200 800

V 100 100 100 100 100 100 2000 100

Sc 26 30 28 30 45 45 100 30

Ce/Yb 1.48 1.00 2.65 1.59 1.54 1.30 1.17 1.76

(B) Calculated partial melts of peridotite and garnet peridotite source compositions, compared to average high-MgO basalt High-MgO basalt (average) Peridotite: 25% melt Garnet Peridotite: 25% melt

Rb 13 11.7 11.8

Sr 405 400.6 405.8

Ba 63 58.7 62.6

Ce 11.2 11.1 11.2

Sm 6 6.1 6.1

Eu 5 5.1 5.0

Yb 5.1 5.2 5.2

Zr 127 126.5 126.3

Ni 96 98.4 100.1

Cr 207 204.5 208.8

V 275 214.9 99.5

Sc 26 26.1 27.1

Ce/Yb 2.20 2.14 2.15

(C) Calculated partial melts of peridotite, garnet peridotite, and garnet granulite source compositions, compared to average low-MgO basalt. Garnet granulite 1=15% modal garnet in residue, garnet granulite 2=25% modal garnet in residue Low-MgO basalt (average) Peridotite: 15% melt Garnet Peridotite: 15% melt Garnet Granulite 1: 10% melt Garnet Granulite 2: 15% melt

Rb 27 25.6 25.7 29.2 22.6

Sr 363 366.5 375.2 351.0 244.8

Ba 107 108.6 95.8 114.5 83.9

Ce 24.7 24.7 25.1 24.6 23.3

Sm 8.6 8.5 8.7 8.6 8.5

Eu 7.4 7.4 7.3 7.5 4.6

Yb 5.1 5.2 5.2 5.1 5.1

Zr 102 186.8 101.9 100.8 77.8

Ni 69 72.7 65.0 68.1 77.5

Cr 182 181.5 181.1 186.3 186.9

V 313 253.8 99.4 65.5 47.0

Sc 28 28.1 26.7 27.5 24.8

Ce/Yb 4.84 4.72 4.86 4.87 4.54

(D) Calculated partial melts of garnet granulite (25% garnet in residue) and garnet amphibolite (45% garnet in residue) source compositions, compared to Group 4 andesite sample DR504. The garnet amphibolite mode re¯ects observed compositions in the

DR 504 (Group 4 andesite) Garnet Granulite 2: 10% melt Garnet Amphibolite: 15% melt

Rb

Sr

Ba

121 125 73

189 121 152

341 310 99

Ce

Sm

Eu

Yb

89.9 90.11 89.94

32.8 32.87 31.63

13.5 15.06 13.28

12.6 12.63 12.18

model quantitatively because the major element systematics re¯ect in part element mobility during metamorphism. Basaltic andesites and andesites of groups 2 and 3 may also represent fractionates of more primitive parent magmas, but without REE data for more of the basalts, it is not possible to link speci®c parent magmas to each group. The REE data for the basaltic andesites and andesites of these two groups show that the source regions of their parent magmas were enriched in LREE, or contained signi®cant refractory garnet. The small negative Eu anomalies in these rocks shows that either plagioclase was not an important phase during fractional crystallization, or (more likely) that oxygen fugacities were relatively high and Eu2+/ Eu3+ ratios relatively low. Forward modeling of major element fractionation using the simulation program Comagmat (Ariskin et al., 1993) shows that most major elements are consist-

Zr

Ni

Cr

V

Sc

Ce/Yb

159 89 156

54 53 44

59 62 56

134 46 119

16 16 17

7.13 7.13 7.39

ent with simple fractionational crystallization of the most primitive basalts (both high-MgO and low-MgO) and basaltic andesites, given the observed scatter due to metamorphism (Fig. 6). These results, however, are inconsistent with the trace element data discussed above, which show that several di€erent parent magmas must be involved (e.g. Fig. 9). Major element mixing models (not shown) suggest that some andesites may form by magma mixing of low-MgO basalt and rhyolite or dacite, but these mixtures do not have appropriate trace element concentrations to represent observed andesite compositions. In particular, compatible elements like Cr, Ni, and V have higher concentrations in the andesites than in calculated mixtures. The single group 4 andesite (DR 504) is distinguished from rocks of the other groups by several important characteristics: extremely high La and La/Lu ratio, high Rb and K2O, and a deep negative Eu anomaly. The intermediate silica content and moderate

M. Tahir Shah, J.W. Shervais / Journal of Asian Earth Sciences 17 (1999) 459±475

Cr and Ni concentrations are inconsistent with extreme fractional crystallization, and suggest a parent magma with chemical characteristics similar to the whole rock. The high La/Lu ratio implies either a source strongly enriched in LREE, or one which is rich in refractory garnet. The high Rb and K2O suggest a crustal source. Partial melting calculations are consistent with 10± 15% melting a ma®c crustal source (Ce/Yb =1.2± 1.7  chondrite) leaving either a garnet granulite or garnet-amphibolite residue (Table 3). These residues are mineralogically similar to high-pressure granulites of the Jijal complex, which forms the lowermost crustal unit of the Kohistan terrane (Jan, 1979a,b,c; Jan and Howie, 1981). 6.3. Metamorphism The Dir-Utror metavolcanic sequence contains two distinct metamorphic assemblages which correspond to a prograde metamorphic trajectory: 1. albite-oligoclasess+epidote+Fe-chlorite+actinolite+/ÿ muscovite 2. albite-andesiness+epidote+Mg-chlorite+tschermakitic hornblende The ®rst assemblage, which corresponds to the greenschist facies, is dominant. Metavolcanic rocks with this assemblage are widely distributed throughout the area and include both ma®c and felsic varieties. The latter assemblage, which corresponds to the epidote amphibolite facies, overprints greenschist facies assemblages locally (e.g. actinolites with blue-green amphibole rims) and is only rarely the dominant assemblage in a given sample. Epidote amphibolite assemblages are generally localized along sheared zones and within the sheared metavolcanic unit. This distribution suggests that higher metamorphic temperatures or increased reaction progress are associated with hydrothermal ¯uid ¯ow along shear zones, which act as conduits for the circulating ¯uids. The occurrence of albite-oligoclasess with actinolite, and the low contents of Alvi and Na(M1) in amphibole are consistent with metamorphism along high geothermal gradients at relatively low total pressure (Miyashiro, 1973; Raase, 1974; Brown, 1977; Maruyama et al., 1983; Ishizuka, 1985; Laird and Albee, 1981). The presence of pure chlorite (without interlayered smectite) and the assemblage actinolite+chlorite+albite+quartz suggests minimum metamorphic temperatures higher than 3008C (Nitsch, 1971; Ewarts and Schi€man, 1983). Liou et al. (1974) suggest 4758C as upper thermal boundary for the typical greenschist assemblage (albite+epidote+chlorite+actinolite) and 5508C as lower thermal boundary for the amphibolite assemblage (calcic-plagioclase+-

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hornblende) at Pf =Pt=2 kb and oxygen fugacity of quartz+fayalite-magnetite (QFM) bu€er. Transitional mineral assemblages remain stable within the temperature interval between 475±5508C during these experiments (Liou et al., 1974). The patchy distribution of low-grade (epidote) amphibolite assemblages in the Dir meta-volcanics implies that the maximum temperatures attained during metamorphism were in this range (ca 5008C). 7. Conclusions The Dir-Utror volcanics represent a continental margin arc assembled along the southern border of Asia after collision of the Mesozoic Kohistan island arc and its subsequent amalgamation to the mainland. Detailed geologic mapping shows that in the region around Dir the Dir-Utror volcanic series is dominated by ma®c to intermediate composition rocks derived from LREE-enriched mantle beneath the arc. The high proportion of high-MgO basalts (12% areally) is similar to that observed in the Aleutian arc (Myers, 1988). The scarcity of more evolved felsic volcanics (dacite, rhyolite) can be explained by the nature of the underlying crust, which consists of accreted intra-oceanic arc volcanic and plutonic rocks, and is ma®c relative to normal continental margins. Most felsic volcanics (rhyolites, dacites) have REE systematics that are consistent with the hypothesis that they formed by fractional crystallization of more ma®c basaltic andesites. Magma-mixing of low-MgO basalt with rhyolite or dacite does not seem to be important in this volcanic series, although this process appears to be common in the southern Andes (Hickey-Vargas et al., 1989). Some andesites may have formed as crustal melts, based on their high LILE contents, high La/Lu, and deep negative Eu anomalies. The REE pattern for one of these andesites (DR504) crosses the chondritenormalized patterns of dacites and rhyolites, showing that these rocks cannot be related by fractional crystallization, assimilation, or magma-mixing. The REE systematics of these andesites are compatible with an origin by crustal anatexis, leaving a refractory residue mineralogically similar to high pressure ma®c and ultrama®c granulites of the Jijal complex. Acknowledgements This paper represents part of a Ph.D. dissertation completed by M.T. Shah (1991) at the University of South Carolina. This research was supported by National Science Foundation grant INT88-13655 to Shervais, and by a U.S. AID scholarship to Shah. ICP analyses were carried out at Eastern Washington

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