扬子陆块西缘江浪穹窿超基性岩的成因:锆石U-Pb定年、岩石地球化学及Sr-Nd同位素

Petrogenesis of the ultramafic pluton in the Jianglang dome, western margin of the Yangtze block: Zircon U-Pb dating, geochemistry and Sr-Nd isotopes

关键词:  超基性岩  锆石U-Pb定年  地球化学  Sr-Nd同位素  江浪穹窿

Key words:  ultramafic pluton  zircon U-Pb dating  geochemistry  Sr-Nd isotopes  Jianglang dome

摘要:

江浪穹窿位于扬子陆块西缘，本文作者在穹窿南部新发现一套侵入于二叠系及志留系的超基性岩，岩石主要由蛇纹石（约60%）、橄榄石（约30%）和少量磁铁矿（约5%）、角闪石（约5%）组成。为探讨超基性岩的成因，本文进行了LA-ICP-MS锆石U-Pb定年、岩石地球化学及Sr-Nd同位素研究。定年结果表明，超基性岩中发育大量2427~430 Ma的捕获锆石，最年轻一组岩浆锆石²⁰⁶Pb/²³⁸U加权平均年龄为222.3±4.4 Ma（MSWD=1.9，n=6）。主微量元素分析显示岩石：（1）具有低的SiO₂含量（46.76%~39.07%）、高的Mg#值（82.3~74.0）与Cr、Co、Ni丰度；（2）稀土元素含量（ΣREE平均31.8 μg/g）与（La/Yb）_N值（5.26~1.38）偏低，稀土配分型式较为平坦，具有较弱的Ce负异常（Ce/Ce^*=0.80~0.67）；（3）富集大离子亲石元素Rb、Ba和U，亏损高场强元素Zr和Hf；（4）（Th/Yb）_PM值（29.8~1.56）、（Th/Ta）_PM值（0.22~0.03）、（La/Nb）_PM值（1.91~0.39）及La/Sm值（5.88~1.11）较低。Sr-Nd同位素分析显示，超基性岩具有较低的（⁸⁷Sr/⁸⁶Sr）_i值（0.706872~0.702598）和高的ε_Nd（t）值（8.02~5.64），成分接近于亏损地幔和岛弧玄武岩，计算表明地壳物质的混染程度低于5%。结合前人研究成果，本文认为超基性岩结晶年龄为222.3 Ma，可能形成于古特提斯洋闭合阶段的岛弧背景；原始岩浆来自高度部分熔融的地幔源区，上升侵位过程中可能经历了铬铁矿与橄榄石的分离结晶作用。此外，捕获锆石的年龄谱反映江浪穹窿很可能存在太古宙—古元古代变质基底，并且具有Rodinia超大陆会聚—裂解以及泛非事件的地质年龄记录。

Abstract:

The Jianglang dome is located in the western margin of the Yangtze block.A set of ultramafic pluton were newly discovered in the south of the Jianglang dome and they intruded into the Permian and Silurian. The rocks are mainly composed of serpentine (ca. 60%), olivine (ca. 30%) with minor magnetite (ca. 5%) and hornblende (ca. 5%). In order to discuss the petrogenesis, we performed LA-ICP-MS zircon U-Pb dating, geochemical and Sr-Nd isotopic studies. Dating results show that the ultramafic pluton have numerous 2427~430 Ma captured zircons, and ²⁰⁶Pb/²³⁸U weighted mean age of the youngest magmatic zircons is 222.3±4.4 Ma (MSWD=1.9, n=6). Geochemical analyses results show that: (1) the rock samples exhibit low SiO₂contents (46.76%~39.07%) with high Mg# values (82.3~74.0) and Cr, Co, Ni abundances; (2) low ΣREE (average 31.8 μg/g) and (La/Yb)_N values (5.26~1.38) with flat REE pattern and negative Ce anomalies (Ce/Ce^*=0.80~0.67); (3) enrichment in large ion lithophile elements (e.g., Rb, Ba and U) and depletion in high field strength elements (e.g., Zr and Hf); (4) low (Th/Yb)_PM values (29.8~1.56), (Th/Ta)_PM values (0.22~0.03), (La/Nb)_PM values (1.91~0.39) and La/Sm values (5.88~1.11). Sr-Nd isotopic analyses indicate that the samples have low (⁸⁷Sr/⁸⁶Sr)_ivalues (0.706872~0.702598) and high ε_Nd(t) values (8.02~5.64). Their compositions are close to the depleted mantle and island arc basalts with contamination of crustal materials lower than 5%. Combining with the previous studies, we propose that the crystallization age of ultramafic pluton is ca. 222.3 Ma which probably formed under island arc setting of the Paleo-Tethys closure stage. The primitive magma was possibly derived from highly partial melting of mantle source region. The magma might experience crystallization differentiation of chromite and olivine. In addition, the age spectrum of captured zircons reflects that Archean-Palaeoproterozoic metamorphosed basements most likely exist in the Jianglang dome, where also have geological age records of the Rodinia supercontinent convergence-disintegration processes and Pan-African events.