藏南也拉香波早渐新世富钠过铝质淡色花岗岩的成因机制及其构造动力学意义

Early Oligocene Na-rich peraluminous leucogranites in the Yardoi gneiss dome, southern Tibet: Formation mechanism and tectonic implications.

藏南也拉香波穹隆位于近东西向展布的北喜马拉雅片麻岩穹隆(NHGD)最东端,主要由石榴角闪岩、石榴石云母片麻岩、二云母花岗岩和淡色花岗岩组成。SHRIMP锆石U/Pb定年结果表明也拉淡色花岗岩的结晶年龄为35.3±1.1Ma,明显老于位于该穹隆以西类似的淡色花岗岩(年龄普遍<25Ma)。全岩元素和Sr-Nd同位素测试结果揭示:(1)也拉香波淡色花岗岩为过铝质富钠花岗岩;(2)与片麻岩相似,也拉香波淡色花岗岩富集大离子亲石元素(LILE,如K,Sr,Rb和Ba),但亏损Ti,Y,Yb,Sc和Cr;(3)和片麻岩或角闪岩相比,也拉香波淡色花岗岩同时亏损LREE和HREE,但与HREE相比,LREE相对富集;(4)在Sr-Nd同位素系统特征上,淡色花岗岩初始Sr同位素比值与角闪岩的相当,在0.711949～0.719344之间;但远小于片麻岩。而Nd同位素组成在片麻岩和角闪岩之间,在-8.9～-15.0之间。以石榴角闪岩和片麻岩为瑞元,简单混合计算表明:由石榴角闪岩为主和片麻岩为辅组成的混合源区发生部分熔融作用,各自产生的熔体进行不同程度的混合,可形成类似于也拉香波淡色花岗岩成分的岩浆,其中角闪岩的部分熔融起主要作用。使用Zr在岩浆中的饱和浓度温度计得出岩浆的平均温度为673℃,在此温度下,变泥质片麻岩在高压(～10kbar)条件下的水致部分熔融和角闪岩部分熔融都可形成也拉过铝质富钠淡色花岗岩,但角闪岩的脱水部分熔融起主导作用。在地壳增厚条件下,下地壳角闪岩的部分熔融可能是导致喜玛拉雅造山带从缩短增厚向伸展垮塌转换的主要因素之一。

The east-west trending Northern Himalayan Gneiss Dome (NHGD) consists of a series Of Semi-continuous gneiss domes within the Tethyan Himalayan Belt. The Yardoi gneiss dome, the easternmost one among these domes, consists of garnet two-mica gneiss, garnet amphibolite, two-mica granite and leucogranite. Available SHRIMP zircon U/Pb data show that the Yardoi leucogranite formed at 35.3 +/- 1.1Ma, older than those similar leucogranites to the west (commonly with ages <25Ma). Bulk element and Sr and Nd isotope analyses show that : (1) a majority of the Yardoi leucogranites are Na-rich and peraluminous; (2) similar to the wall-rock gneisses, the Yardoi Ieucogranites are enriched in LILE (e.g. K, Sr, Rb, and Ba), but highly depleted in Ti, Y, Yb, Sc, and Cr; (3) as compared to those either in the gneisses or in the amphibolites, the Yardoi leucogranites are depleted both in LREE and in HREE. However, they are enriched in LREE relative to HREE; (4) the Yardoi leucogranites have initial Sr isotope compositions ranging from 0.71195 to 0.71934, similar to those in the amphibolites, whereas they have relatively lower epsilon(Nd)(i) values ( -8.9 similar to 15. 0), which is intermediate between those in the amphibolites and the gneisses. Two end member simple mixing calculations indicate that partial melting of a source mixed of garnet amphibolite with subordinate pelitic gneiss could account for the formation of the Yardoi Na-rich peraluminouis melts. The Yardoi leucogranites have an average Zr saturation temperature of 673 degrees C. At this temperature, both H(2)O-fluxed melting of metapelite at high-pressures ( similar to 10kbar) and amphibolite parting melting can produce Na-rich peraluminous granitic melts. However, amphibolite partial melting dominated the similar to 35 Ma anatectic event in the Yardoi gneiss dome. This event might be a major factor that led to the transition from compressional to extensional deformation in the overthickened Himalayan belt.