ABSTRACT

Granitoids in active continental margins are probes of sub-arc magmatic processes and are crucial for the understanding of crust–mantle interaction during subduction. Here we report data for the middle to late Cambrian Tuole (TL) and Dabaishitou (DBST) granitic plutons in the western segment of the North Qilian suture zone (NQSZ). The TL-DBST plutons comprise monzonite, alkali-feldspar granite, gabbroic diorite, monzonitic porphyry and biotite monzogranite. Zircon U‒Pb ages reveal that these granitoids crystallized at ca. 509‒492 Ma, consistent with the early subduction of the North Qilian Ocean. Mineralogical, petrological, and geochemical features indicate that the TL monzonite and gabbroic diorite are both high-K calc-alkaline I-type granitoids but the latter is Nb-enriched; the TL alkali-feldspar granites and DBST granitoids belong to aluminous A-type and transitional I- and A-type granites, respectively. The I- and aluminous A-type granites show similar and comparable isotopic compositions, e.g., ε_Hf(t) values of ‒5.6 to +2.7 versus ‒2.4 to +3.5, but different melting conditions, e.g., high temperature and low oxygen fugacity for aluminous A-type granites as indicated by zircon trace elements. Geochemical and isotope data suggest that all these granitoids were mainly derived from the sub-arc mantle metasomatized by slab-derived fluids/asthenospheric mantle-derived melts/both prior to melting with minor input of LCC-derived melts (ca. 10–20%). The HFSE enrichment feature was inherited by granites and further enhanced by protracted crystal fractionation at low oxygen fugacity conditions to produce the TL aluminous A₁-type granites but weakened by fractionation at relatively high oxygen fugacity conditions to produce the DBST I-A-transitional-type granite. Partial melting of the sub-arc lithospheric mantle was induced by upwelling of the asthenospheric mantle and their interactions in response to slab rollback during subduction initiation (ca. 509–506 Ma), and by addition of slab-derived fluids again as subduction proceeded (ca. 493–492 Ma).

摘要

活动大陆边缘的花岗岩是亚弧岩浆过程的探测器，对于理解俯冲过程中的壳幔相互作用至关重要。在这里，我们报告了北祁连缝合带 (NQSZ) 西段中晚寒武世托勒 (TL) 和大白石头 (DBST) 花岗岩岩体的数据。TL-DBST 岩体包括二长岩、碱长石花岗岩、辉长闪长岩、二长斑岩和黑云二长花岗岩。锆石 U-Pb 年龄表明这些花岗岩在大约 509-492 Ma，与北祁连洋早期俯冲一致。矿物、岩石和地球化学特征表明，TL 二长岩和辉长闪长岩均为高钾钙碱性 I 型花岗岩，后者富含 Nb；TL碱长石花岗岩和DBST花岗岩分别属于铝质A型和过渡I型和A型花岗岩。I 型和铝质 A 型花岗岩显示出相似和可比的同位素组成，例如，ε_Hf (t) 值分别为 ‒5.6 至 +2.7 与 ‒2.4 至 +3.5，但熔融条件不同，例如锆石微量元素表明铝 A 型花岗岩的高温和低氧逸度。地球化学和同位素数据表明，所有这些花岗岩类主要来自亚弧地幔，由板片衍生流体/软流圈地幔衍生熔体/两者在熔化之前与少量 LCC 衍生熔体（约 10-20%）交代）。HFSE 富集特征由花岗岩继承，并通过在低氧逸度条件下延长晶体分馏得到 TL 铝质 A ₁进一步增强型花岗岩，但在相对较高的氧逸度条件下通过分馏而减弱，以产生 DBST IA 过渡型花岗岩。亚弧岩石圈地幔的部分熔化是由软流圈地幔的上涌及其相互作用引起的，以响应俯冲开始（约 509-506 Ma）板片回滚，并随着俯冲作用的进行再次加入板片衍生流体（约 493–492 Ma）。