西秦岭凤太矿集区花岗闪长斑岩脉的成因类型和年龄及其地质意义

摘要

The Fengtai ore concentration area in Shaanxi Province is one of the most important lead-zinc and gold ploymetallic ore concentration areas in the West Qinling orogen. In this paper, two samples were collected from the Kongguan and Jintonggou granodiorite porphyry dikes, which are filled in the NWW–trending Tonglinggou–Jiashangou fault in the north central part of the ore concentration area. LA-ICP-MS dating yielded zircon ages of (230.7±1.8) Ma (MWSD=0.68) and (230.4±1.8) Ma (MSWD=0.27), respectively, suggesting Middle Triassic. Geochemical features are as follows: strong depletion of high field strength elements (HFSE), such as Th, Nb, Ta, P and Ti, with Nb/Ta ratios similar to those of the continental crust; strongly fractionated REE patterns ((La/Yb)N=250.69 and 134.94) and no notable Eu anomalies; the 87Sr/86Sr ratios of the two samples being 0.712 833±0.000 017 and 0.712 365±0.000 016. All these data indicate that the magma might have been derived from partial melting of the Neoproterozoic Bikou Group, and there existed garnet with no plagioclase left in the source. The dikes were formed at the beginning of the syn-collision period. By integrating the regional tectonic and magmatic characteristics, the authors hold that tectonic evolution of the ore concentration area can be divided into the subduction stage (±250 Ma), the collision stage (±230 Ma) and the post-collision stage (220–200 Ma). The age of 230 Ma from these two dikes gives an upper time limit of the NWW–trending faults in the ore concentration area, implying they were formed during the middle Triassic, earlier than the NE–trending fractures which were formed during the late Triassic. More and more age data show that the time of deformation, metamorphism, magmatism and mineralization in the Fengtai ore concentration area are all focused on 190–230 Ma, implying they are spatial-temporal coupling with each other, and were formed during the Qinling orogenic process in Indosinian period.%凤太矿集区是西秦岭最重要的金、铅锌、铜多金属成矿区之一.本次工作对产于矿集区中北部NWW 向断裂带内的孔棺和金铜沟花岗闪长斑岩脉进行了 LA-ICP-MS 锆石 U-Pb 定年, 获得(230.7±1.8) Ma和(230.4±1.8) Ma的年龄, 属中三叠世.两岩脉稀土元素配分呈右倾型, 轻、重稀土强烈分馏, 无明显 Eu 异常, 亏损高场强元素, Nb/Ta 比值接近陆壳; 87Sr/86Sr 同位素比值分别为0.712833±0.000017和0.712365±0.000016.地球化学分析表明成岩物质以壳源为主, 岩浆可能主要来自扬子板块北缘碧口群的部分熔融, 源区存在石榴子石等矿物残留, 无斜长石残留, 形成于同碰撞初始阶段.结合区域构造和岩浆特征, 认为凤太矿集区经历了250 Ma左右的洋壳俯冲阶段、230 Ma左右的同碰撞阶段以及220—200 Ma的后碰撞阶段.岩脉230 Ma的年龄限定了区内NWW向断裂带活动的时间上限.区内NWW向构造约形成于中三叠世, NE向构造发育于晚三叠世.区域构造、岩浆及金、铅锌多金属成矿的时空耦合, 暗示了三者都是印支期秦岭造山作用过程的产物.