P–T path of high‐pressure/low‐temperature rocks and tectonic implications in the western Tianshan Mountains, NW China

The Chinese western Tianshan high‐pressure/low‐temperature (HP–LT) metamorphic belt, which extends for about 200 km along the South Central Tianshan suture zone, is composed of mainly metabasic blueschists, eclogites and greenschist facies rocks. The metabasic blueschists occur as small discrete blocks, lenses, bands, laminae or thick beds in meta‐sedimentary greenschist facies country rocks. Eclogites are intercalated within blueschist layers as lenses, laminae, thick beds or large massive blocks (up to 2 km2 in plan view). Metabasic blueschists consist of mainly garnet, sodic amphibole, phengite, paragonite, clinozoisite, epidote, chlorite, albite, accessory titanite and ilmenite. Eclogites are predominantly composed of garnet, omphacite, sodic–calcic amphibole, clinozoisite, phengite, paragonite, quartz with accessory minerals such as rutile, titanite, ilmenite, calcite and apatite. Garnet in eclogite has a composition of 53–79 mol% almandine, 8.5–30 mol% grossular, 5–24 mol% pyrope and 0.6–13 mol% spessartine. Garnet in blueschists shows similar composition. Sodic amphiboles include glaucophane, ferro‐glaucophane and crossite, whereas the sodic–calcic amphiboles mainly comprise barroisite and winchite. The jadeite content of omphacite varies from 35–54 mol%. Peak eclogite facies temperatures are estimated as 480–580 °C for a pressure range of 14–21 kbar. The conditions of pre‐peak, epidote–blueschist facies metamorphism are estimated to be 350–450 °C and 8–12 kbar. All rock types have experienced a clockwise P–T  path through pre‐peak lawsonite/epidote‐blueschist to eclogite facies conditions. The retrograde part of the P–T  path is represented by the transition of epidote‐blueschist to greenschist facies conditions. The P–T  path indicates that the high‐pressure rocks formed in a B‐type subduction zone along the northern margin of the Palaeozoic South Tianshan ocean between the Tarim and Yili‐central Tianshan plates.

[1]  N. Volkova,et al.  Geochemical discrimination of metabasalt rocks of the Fan Karategin transitional blueschist/greenschist belt, South Tianshan, Tajikistan: seamount volcanism and accretionary tectonics , 1999 .

[2]  S. Maruyama,et al.  Blueschists and Eclogites of the World and Their Exhumation , 1996 .

[3]  M. Tagiri,et al.  Mineral parageneses and metamorphic P-T paths of ultrahigh-pressure eclogites from Kyrghyzstan Tien-Shan , 1995 .

[4]  X. Xiao,et al.  The mineralogy, petrology, metamorphic PTDt trajectory and exhumation mechanism of blueschists, south Tianshan, northwestern China , 1995 .

[5]  T. Hirajima,et al.  Petrology of ultrahigh-pressure rocks from the southern Su-Lu region , 1995 .

[6]  S. Graham,et al.  Late Oligocene-early Miocene unroofing in the Chinese Tian Shan: An early effect of the India-Asia collision , 1994 .

[7]  J. Liou,et al.  Coesite-bearing eclogite in Henan Province, central China; detailed petrography, glaucophane stability and PT-path , 1994 .

[8]  J. Liou,et al.  Polyphase and anticlockwise P‐T evolution for Franciscan eclogites and blueschists from Jenner, California, USA , 1994 .

[9]  M. Allen,et al.  Palaeozoic collisional tectonics and magmatism of the Chinese Tien Shan, central Asia , 1993 .

[10]  Xiaoming Wang,et al.  Coesite-Bearing Eclogites from the Dabie Mountains, Central China: Petrogenesis, P-T Paths, and Implications for Regional Tectonics , 1992, The Journal of Geology.

[11]  N. Dobretsov.  Blueschists and eclogites : a possible plate tectonic mechanism for their emplacement from the upper mantle , 1991 .

[12]  B. W. Evans.  Phase relations of epidote-blueschists , 1990 .

[13]  R. A. Howie.  Evolution of Metamorphic Belts , 1990, Mineralogical Magazine.

[14]  S. Banno,et al.  Prograde and retrograde metamorphism of hematitebearing basic schists in the Sanbagawa belt in central Shikoku , 1990 .

[15]  H. Day,et al.  Controls on the first appearance of jadeitic pyroxene, northern Diablo Range, California , 1989 .

[16]  R. Coleman,et al.  Blueschists in major suture zones of China , 1989 .

[17]  R. Coleman.  Continental growth of northwest China , 1989 .

[18]  D. Moore,et al.  New evidence for polyphase metamorphism of glaucophane schist and eclogite exotic blocks in the Franciscan Complex, California and Oregon , 1989 .

[19]  W. Ernst.  Tectonic history of subduction zones inferred from retrograde blueschist P-T paths , 1988 .

[20]  Nobuo Morimoto,et al.  Nomenclature of Pyroxenes , 1988, Mineralogical Magazine.

[21]  H. Massonne,et al.  Phengite geobarometry based on the limiting assemblage with K-feldspar, phlogopite, and quartz , 1987 .

[22]  A. Barnicoat,et al.  High-pressure metamorphism of the Zermatt-Saas ophiolite zone, Switzerland , 1986, Journal of the Geological Society.

[23]  F. Spear,et al.  Aluminous titanite (sphene) from the Eclogite Zone, south-central Tauern Window, Austria , 1985 .

[24]  T. Holland.  The experimental determination of activities in disordered and short-range ordered jadeitic pyroxenes , 1983 .

[25]  R. Kretz.  Symbols for rock-forming minerals , 1983 .

[26]  B. Leake,et al.  Nomenclature of Amphiboles , 1978, Mineralogical Magazine.

[27]  W. Maresch.  Experimental studies on glaucophane: An analysis of present knowledge , 1977 .

[28]  Gao Ju,et al.  Formation Environment of the High-Pressure Metamorphic Rocks, Western Tianshan, Xinjiang, China , 1997 .

[29]  M. Scambelluri,et al.  Retrograde P‐T‐t path for the Voltri Massif eclogites (Ligurian Alps, Italy): some tectonic implications , 1991 .

[30]  C. Biino,et al.  Petrology of very-high-pressure eclogitic rocks from the Brossasco-Isasca Complex, Dora-Maira Massif, Italian Western Alps , 1991 .

[31]  M. Schliestedt.  Occurrences and stability conditions of low-temperature eclogites , 1990 .

[32]  N. Dobretsov,et al.  Eclogites from various types of metamorphic complexes in the USSR and the problems of their origin , 1986 .

[33]  P. Black,et al.  Regional eclogite facies in the high-pressure metamorphic belt of New Caledonia , 1986 .

[34]  E. Brown,et al.  Blueschists and eclogites , 1986 .

[35]  N. Dobretsov,et al.  Glaucophane schists and eclogites in the folded systems of northern Asia , 1984 .

[36]  C. Burrett.  Plate tectonics and the fusion of Asia , 1974 .

[37]  山口 悟郎.  Na 2 O-Al 2 O 3 系焼結反応について , 1954 .