浙东南玉苍山-彭溪碱长花岗岩锆石U-Pb年龄学、岩石地球化学特征及其成因类型

玉苍山-彭溪岩体位于东部沿海花岗岩带北端,受古太平洋板块俯冲制约。笔者等以该岩体碱长花岗岩为研究对象,开展锆石U-Pb测年,获得其加权平均年龄为96.50±1.81Ma和98.27±0.39 Ma,表明该岩体为晚白垩世岩浆活动的产物。该岩体主要矿物组成为钾长石（60%～70%）、石英（25%～35%）,少量斜长石（＜5%）、黑云母（＜2%）等,呈中细粒花岗结构。该岩体地球化学特征上呈现高硅、富碱、低铝,镁、钙、磷、钛、铁等元素含量极低,Rb、K等大离子亲石元素和Th、U、Zr、Hf等高场强元素富集,亲铁元素Co、Ni、Cr、V等含量较低;K/Rb值低,Rb/Sr、Rb/B值高;稀土元素总量高,且轻稀土元素富集程度远大于重稀土元素,同时具有明显的铕负异常,分异指数高;反映了该岩体具有强烈的壳源特征,且形成时岩浆演化分异程度高,存在斜长石、磷灰石等矿物的分离结晶,属于过铝质高钾钙碱性花岗岩系列。认为该岩体属于A型花岗岩,为晚白垩世时期板内伸展构造环境下软流圈地幔上涌、部分地壳参与熔融形成的。

Zircon U-Pb geochronology, rock geochemical characteristics and genetic types of the Yucangshan-Pengxi Alkali Feldspar Granite in Southeastern Zhejiang

Objectives: The Yucangshan-Pengxi pluton is geographically located in the southeastern part of Zhejiang Province, at the northern end of the granite belt along the eastern coast of China. Magmatic activity is constrained by the subduction of the Pacific Plate, and is also controlled by the Wenzhou Zhenhai Fault in the region, trending northeast.Methods: Based on the field investigation, We conducted microscopic observations, whole-rock geochemical analysis, and LA-ICP-MS zircon U-Pb dating for the Yucangshan-Pengxi alkali feldspar granite.Results: Two sets of high-precision dating data were obtained, with weighted average ages of 97.0 ± 1.8 Ma and 98.27 ± 0.39 Ma, respectively, indicating that the rock mass is a product of Late Cretaceous magmatic activity. Under the microscope, the main mineral composition of the rock mass is potassium feldspar (60%~69%), quartz (25%~35%), as well as a small amount of plagioclase (<5%), biotite (<2%), etc., presenting a medium to fine-grained granite structure. The rock mass exhibits high Si, rich alkali, low Al; depleted in Mg, Ca, P, Ti, Fe. The samples are enriched in large ion lithophilic elements such as Rb, K, etc; as well as high field strength elements such as Th, U, Zr, and Hf, are enriched. The content of ferrophilic elements such as Co, Ni, Cr, and V is relatively low; The K/Rb ratio is low, while the Rb/Sr and Rb/Ba ratios are high; The total amount of rare earth elements is high, and the enrichment degree of light rare earth elements is much higher than that of heavy rare earth elements. At the same time, it has obvious europium negative anomalies and a high differentiation index; This reflects the strong crustal characteristics of the rock mass, and the high degree of differentiation in magma evolution during its formation.Conclusions: During the diagenesis process, minerals such as plagioclase and apatite were separated and crystallized, belonging to the peraluminous high potassium calcium alkaline granite series. Through exploration of the rock type and tectonic environment, it is believed that the rock mass belongs to A-type granite, which was formed by the upwelling of the asthenosphere mantle and partial melting of the crust in the late Cretaceous intraplate extensional tectonic environment.