大兴安岭北部塔河花岗杂岩体的地球化学特征及成因

Geochemical characteristics and genesis of Tahe granitic complex in northern part of the Da Hinggan Range.

塔河杂岩体位于塔源-喜桂图缝合带北侧的额尔古纳地块东部,是早古生代侵入的花岗杂岩体。该杂岩体的主要岩石类型为正长花岗岩、二长花岗岩,少量碱长花岗岩和花岗闪长岩,辉长岩以包体存在于花岗岩中。岩石成因类型为典型的Ⅰ型后造山侵入体。岩体在野外地质特征、矿物组合、显微结构、化学成分及锆石Hf同位素特征等方面都表现出岩浆混合成因。在早古生代额尔古纳地块与兴安地块拼合后的后造山伸展拉张背景下,地壳和地幔都发生部分熔融,直接起源于亏损地幔的玄武质岩浆侵入到下地壳熔融的花岗质岩浆房,经结晶分异作用,形成了塔河杂岩体不同的岩石类型。花岗岩的εHf(t)为-0.8^+5.6之间,Hf模式年龄在0.9~1.5Ga之间,反映塔河花岗岩的源岩应该是在中-新元古代时期由亏损地幔起源的新生地壳物质。结合额尔古纳地块早古生代和中生代花岗岩锆石Hf同位素资料,我们认为额尔古纳地块在中-新元古代时曾发生过一次重要的地壳增生事件。

Tahe pluton is a granitic complex formed in Early Paleozoic,which locates in the eastern Ergun block near the north side of Tayuan-Xiguitu suture zone.It is mainly composed of syenogranite,monzogranite,with minor alkali-feldspar granite and granodiorite.Gabbroic enclaves are enclosed by granitic rocks.The complex is classified as post-orogenic I-type intrusion,with the characteristics of high-K calc-alkaline series.Characteristics of outcrop,microstructures,mineral assemblages,chemical and zircon Hf isotopic compositions show magma mixing during the petrogenesis definitely.At Early Paleozoic,the crust and mantle underwent partial melting under an extensional environment following the collision between the Ergun and the Xing'an blocks.The basaltic magma which originated from depleted mantle mixed with granitic magma melted by lower crust,and then formed different rock types of Tahe complex through fractionation crystallization.The εHf(t) values(from-0.8 to +5.6) and Hf model ages(0.9~1.5Ga) of the granites indicate that the source rocks of Tahe granite were juvenile crustal materials mainly originated form depleted mantle in Mid-Neo Proterozoic.Combined with previously published zircon Hf isotopic data of the Early Paleozoic and Mesozoic granites in the Ergun block,it is concluded that Ergun block underwent an important episode of crustal growth in Mid-Neo Proterozoic.