兴安地块东南缘晚石炭世侵入岩的锆石U-Pb年代学、地球化学特征及构造意义

Zircon U-Pb ages and geochemical characteristics of Late Carboniferous plutons in the southeastern Xing’an block and their tectonic implications

有关兴安地块与松嫩地块拼合的时限目前仍存较大争议。本文对兴安地块东南缘的新晟、新立与泥鳅河中-酸性侵入岩开展了同位素年代学、岩石学及地球化学分析,进一步限定该区晚古生代构造-岩浆演化。LA-ICP-MS锆石测年结果显示所有岩体均形成于晚石炭世(315~298 Ma)。其中,新立二长花岗岩年龄为304±2 Ma,新晟二长花岗岩、二长岩的年龄分别为312±2 Ma与303±2 Ma,泥鳅河碱长花岗岩、辉长闪长岩年龄分别为315±2 Ma与298±6 Ma。地球化学分析表明这些岩体的ACNK值均大于1,属于高钾钙碱性或钾玄岩系列。泥鳅河辉长闪长岩MgO含量高,Mg值为64~68,Al2O3和TiO2含量较低,表现出赞岐岩的地球化学特征。泥鳅河碱长花岗岩富SiO2、K2O,贫CaO,具有轻稀土富集、Eu强烈负异常的"海鸥"式稀土配分特征,富集大离子亲石元素,属于A2型花岗岩。新立二长花岗岩与新晟二长花岗岩均富K2O,轻、重稀土分异明显,中度亏损Nb、Ta、P、Ti等,具有高分异I型花岗岩的特征。另外,新晟二长花岗岩也表现出高Sr/Y类花岗岩特征(Sr/Y=132~140)。岩石成因研究表明新晟二长花岗岩与泥鳅河碱长花岗岩形成于下地壳部分熔融;新晟二长岩与新立二长花岗岩形成于壳幔岩浆混合作用;泥鳅河辉长闪长岩来源于被俯冲沉积物熔融形成的熔体交代过的地幔部分熔融。通过对比兴安地块早石炭世与晚石炭世岩浆作用的地球化学特征,并结合区域地质资料,本文认为兴安地块和松嫩地块在晚石炭世早期(315 Ma)之前已经完成拼合,晚石炭世侵入岩均形成于后碰撞伸展环境。

The amalgamation timing of the Xing’an and Songneng Blocks is still on controversy. The Xinsheng, Xinli and Niuqiuhe intermediate to felsic plutons in the southeast of Xing’an Block were systematically studied using geochronology, petrology and geochemical analysis, with the aim to understand its tectonic and magmatic evolution during the Late Paleozoic. Zircon LA-ICP-MS U-Pb dating results suggest these plutons formed in the Late Carboniferous(315~298 Ma). The Xinli monzogranite has a zircon U-Pb age of 304±2 Ma. The Xinsheng monzogranite and monzonite formed at 312±2 Ma and 303±2 Ma, respectively. The Niqiuhe alkali-feldspar granite and gabrroic diorite were formed at 315±2 Ma and 298±6 Ma, respectively. Geochemical analysis shows that these rocks are high-K calc-alkaline/shoshonite series and metaluminous. The Niuqiuhe gabbroic diorite has high MgO content(6.8%~8%) and Mg values(64~68), low Al2O3 and TiO2 contents, with affinity of sanukite. The Niqiuhe alkali-feldspar granite has high SiO2 and K2O and low CaO, rich in LREE and LILEs and depleted in Eu, which belongs to A2-type granitoids. The Xinli monzogranite and Xinsheng monzogranite are rich in SiO2, strong REE differentiation, and moderate depletion in Nb, Ta, P and Ti, suggesting the characteristics of highly fractionated I-type granitoids. Furthermore, the Xinsheng monzogranite is characterized by high Sr/Y ratios(132~140). The Xinsheng monzogranite and Niqiuhe alkali-feldspar granite formed likely from partial melting of the lower crust, while the Xinsheng mozonite and Xinli monzogranite were hybid melts by crust-mantle derived magma mixing. The Niqiuhe gabbro diorite originated from slab-derived sediment metasomatized mantle. According to the geochemical difference between Early Carboniferous and Late Carboniferous magmatism, along with regional geological data, it is deduced in this study that the Xing’an and Songnen Blocks had amaglmated together before 315 Ma and these Late Carboniferous intrusions formed in a post-collisional setting.