Discovery of late Early Cretaceous diorite porphyrite from the Shamuluo Formation in the Gaize area, Tibet: Response to the northward subduction plate rollback event of Bangongco-Nujiang Tethys Ocean

Abstract

改则地区闪长玢岩位于藏西北班公湖-怒江成矿带西段北缘，呈岩脉状侵位在成岩于浅海环境的沙木罗组地层中，是班公湖-怒江特提斯洋向北俯冲造山消减过程的产物，而其岩石成因、地质意义和深部动力学过程等尚欠缺详细的研究。本文研究了发育于改则地区的闪长玢岩，对其开展了岩相学、锆石LA-ICP-MS U-Pb年代学以及元素和同位素地球化学的研究，对其岩石成因、成岩构造地质背景、深部地球动力学过程等进行了分析和讨论，并对其含矿性进行了评价，为该地区下一步找矿方向提供了初步的设想。锆石U-Pb同位素年代测试显示闪长玢岩具有较多锆石结晶年龄段，相较于研究区出露的沙木罗组碎屑锆石年代学特征，其最新年龄段（117~126Ma）加权平均年龄为120.4Ma，可代表其结晶年龄。元素地球化学结果显示闪长玢岩具有弧岩浆的典型特征，属于高钾钙碱性和准铝质系列岩石，结合锆石微量元素等指标综合判断其属于I型花岗岩类岩石。本文综合全岩Sr-Nd-Pb同位素组成特征等分析认为，闪长玢岩形成于新生玄武质下地壳的部分熔融，岩浆源区残留相以角闪岩相为主，源区物质有少量沉积物组份的参与，侵位过程中分离结晶和地壳混染作用不明显。通过多种方法判别并结合区域构造背景，我们认为闪长玢岩成岩于班公湖-怒江特提斯洋北向俯冲板片折返的地球动力学背景。由于地处俯冲板片后端根部，改则地区闪长玢岩俯冲角度小于成岩于俯冲板片前端同时代的多龙矿集区中酸性岩浆岩，亦小于成岩于相同板片构造位置的103.5Ma的革吉闪长玢岩，暗示120.4~103.5Ma时班公湖-怒江特提斯洋北向俯冲板片仍处于持续折返阶段。由于其俯冲角度较小以及岩浆源区低氧逸度等因素导致其无法成矿。因此建议在多龙矿集区和改则县中间位置寻找可能的与俯冲背景有关的成矿靶区，但是否见矿还应考虑构造、围岩、后期保存等条件的影响。;The diorite porphyrite in Gaize area, located in the northern margin of the western section of the Bangongco-Nujiang metallogenic belt, emplaced into the Shamuluo Formation formed from shallow sea environment. It was the product of the northward subduction of Bangongco-Nujiang Tethys Ocean (BNTO). Taking diorite porphyrite in Gaize area as the research object, this paper studied petrography, zircon U-Pb chronology, elements, and isotope geochemistry, analyzed its petrogenesis and source characteristics, discussed its diagenetic tectonic background, described its deep dynamic process in detail, explained the possible reasons for its non-mineralization, and provided a preliminary idea for the next prospecting direction in this area. Zircon U-Pb isotopic dating shows that diorite porphyrite has many stages of age. Compared with the detrital zircon chronological characteristics of Shamulo Formation in the study area, the weighted average age of the latest age group (117~126Ma) is 120.4Ma, which can represent its crystallizing age. The element geochemical results show that diorite porphyrite is a typical island arc magma with high potassium calc-alkaline characteristics, belonging to quasi-aluminous series. Combined with the characteristics of zircon trace elements, the diorite porphyrite has the characteristics of I-type granitoids. Based on the analysis of the Sr-Nd-Pb isotopic composition of the whole rock, it is considered that the diorite porphyrite was formed in the partial melting of the juvenile basaltic lower crust, the residual facies in the magmatic source are mainly amphibolite facies, with the addition of a small amount of sediment components, and the separation, crystallization and crustal contamination in the emplacement process is not obvious. Through a variety of methods and combined with the regional tectonic background, we present that the diorite porphyrite was formed in the geodynamic background of rollback within the process of northward subduction of the BNTO. Because it is located at the root of the back end of the subducted plate, the subduction angle of the diorite porphyrite in Gaize area is smaller than that of the intermediate acid magmatic rock in the Duolong ore district, which formed from the front of the subducted plate at same time, and is also smaller than that of Geji diorite porphyrite, which is located at the same tectonic location of the subducted plate and formed at 103.5Ma. It is suggested that the northward subduction plate of the BNTO is still in the stage of continuous rollback during 120.4~103.5Ma. It cannot be mineralized because of its small subduction angle and low oxygen fugacity in the magmatic source area. Therefore, it is suggested to look for possible ore-forming targets related to the subduction background between the Duolong ore concentration area and Gaize County, but the influence of structure, surrounding rocks and late preservation should also be considered.