东准噶尔卡拉麦里断裂以南幔源底侵体、“钉合花岗岩体”的发现及其地质意义

Mantle underplated pluton and stitching granite pluton from south side of the Karamaili fault in eastern Junggar: Geochronological,geochemical and Sr-Nd isotopic constraints on their petrogenesis and tectonic implications

新疆富蕴县滴水泉-畜牧办侵入体出露于卡拉麦里断裂以南,呈北西西向带状分布,以碱长花岗岩体为主,也可见规模较小的角闪辉长岩体。碱长花岗岩体的岩石组合为碱长花岗斑岩＋碱长花岗岩,高硅（SiO2=71.07%-76.71%）,富碱（Na2O＋K2O=7.41%-9.07%）、K2O〉Na2O（平均为1.10）,显示出A型花岗岩的特点。角闪辉长岩体涌动侵入于碱长花岗岩体之中,二者接触带附近发育浆混性质的石英闪长岩。辉长岩＋花岗岩的双峰式岩石组合、构造判别图解R2-R1及区域地质背景指示滴水泉侵入体形成于陆内伸展环境,且花岗岩体具有＂钉合岩体＂的作用,穿插了卡拉麦里蛇绿岩带。结合岩体的LA-ICP-MS锆石年龄（碱长花岗岩的206Pb/238U加权平均年龄为321±2Ma,角闪辉长岩的206Pb/238U加权平均年龄为319±3Ma）可知,卡拉麦里洋盆在晚石炭世早期（321Ma）之前已经闭合。同位素及微量元素特征显示,碱长花岗岩为年轻地壳部分熔合融的产物,而角闪辉长岩则来源于亏损的软流圈地幔及俯冲交代的地幔楔物质,代表了同期花岗岩的底侵岩浆演化的产物。辉长岩与花岗岩相似的εNd（t）值及明显的岩浆混合作用表明该区的花岗岩体并非来源于底侵岩浆的高度分异或底侵体的部分熔融,而最可能为底侵体之上的年轻地壳的部分熔融,这一结论与最近一些学者研究的断裂以北的花岗岩体成因机制相同。晚石炭世早期幔源底侵体的发现,为卡拉麦里地区后碰撞花岗岩类的幔源底侵作用提供了可靠的地质依据。

Dishuiquan-Xumuban intrusions containing K-feldspar granite pluton and visible smaller hornblende-gabbro pluton are located in the south side of the Karamaili fault in Fuyun County,Xinjiang,which shows a characteristic of NWW zonal distribution.The K-feldspar granite pluton is made of K-feldspar granite porphyry and K-feldspar granite,whose characteristics is similar with Atype granite,such as high Si O2content（ 71. 07% - 76. 71%）,K2 O 〉 Na2O（ average of 1. 10）. The hornblende-gabbro pluton is surging K-feldspar granite pluton,quartz diorite is visible in the magma mixing zone. According to intrusion regional geology,the bimodal rock associations and R2-R1 diagram, we can find that a Dishuiquan intrusion is formed in intra-continent extension environment and the granite pluton is a stitching body. LA-ICPMS zircon U-Pb age shows that the K-feldspar granite206 Pb /238 U ages is321 ± 2Ma and hornblende-gabbro is 319 ± 3Ma,so the ocean basin of Karamaili has been closed before the early stage Late Carboniferous（ 321Ma）. Isotope and trace element shows that K-feldspar granites are characterized of the young crust,however hornblende-gabbro source is composed of depleted asthenosphere mantle and wedge matasomasised by subducting slab. Even more important,hornblende-gabbros represent underplating magmas of granites from Late Carboniferous. The similar εNd（ t） and obvious magma mixing between gabbro and granite indicate that Dishuiquan K-feldspar granite pluton is not from high differentiation of underplating magmas or partial melting of hornblende-gabbros,which most probably be from the young crust overhead the underplated hornblende-gabbro pluton. The conclusion is consistent with research achievements from granites in the north of Karamaili by some scholars. The finding of underplated hornblende-gabbro pluton in the early stage of Late Carboniferous provides a reliable geological basis for underplating of post-collision granitoids in Kalamaili.