UPb and Sr isotopic studies on granitoids from Taiwan and Chinmen-Lieyü and tectonic implications
UPb and Sr isotopic studies on granitoids from Taiwan and Chinmen-Lieyü and tectonic implications
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DOI：
10.1016/s0040-1951(96)00023-6
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发表时间：
1996-10
期刊：
影响因子：
2.9
通讯作者：
T. Yui;L. Heaman;C. Lan
中科院分区：
文献类型：
--
作者：
T. Yui;L. Heaman;C. Lan
中国台湾和金门-列屿花岗岩类的锆石/独居石U-Pb定年和Sr同位素结果对构造演化和与中白垩世俯冲过程有关的长英质岩浆作用的时间具有重要的制约作用。金门片麻岩（139.4Ma）和花岗岩（100.9Ma）代表了最古老的花岗岩单位，属于以前公认的燕山晚期岩浆活动阶段。台湾大南坳变质杂岩中有一套较年轻的花岗岩类，包括源头山和大浊水花岗岩（87.3Ma）和卡纳根片麻岩（90.2Ma）。这些中白垩纪年龄的年轻花岗岩类套件减轻了假设的晚古生代“东南崖”微大陆在研究区的存在，如其他研究人员提出的。锆石U-Pb继承年龄在1.67 ~ 2.09 Ga之间，表明中国东南沿海和台湾大部分地区存在前寒武纪基底，但研究区以西的岩石没有太古代地壳的证据。在~（87）Sr ~（86）Sr与~（87）Rb ~（86）Sr对比图上，金门和福建沿海花岗岩类的表观年龄为158-167 Ma。虽然假等时线可以解释为一个两个端元混合过程，随后的分离结晶的结果，假等时线的表观年龄，证实了旁证，被假定为在该地区的前一个（燕山早期）构造事件的地壳原岩的花岗岩类的Sr同位素均一化的近似时间。这样的锶同位素均一化过程必须发生在一个大的区域，可能促进变质流体相或大规模的深熔作用。本区白垩纪花岗岩类可能是地壳局部熔融的产物。闽台沿海白垩纪钙碱性花岗岩类为I型或混染I型。在岩浆成因过程中，俯冲分量被推断。虽然台湾花岗岩类被认为与俯冲作用有关，但福建沿海花岗岩类形成的可能构造环境仍有待解决。
UPb zircon/monazite dating and Sr-isotope results for granitoids from Taiwan and Chinmen-Lieyü, SE China place important constraints for the tectonic evolution and the timing of felsic magmatism associated with mid-Cretaceous subduction process. The Chinmen gneiss (139.4 Ma) and granite (100.9 Ma) represent the oldest granitic units and belong to the previously recognized late Yanshanian phase of magmatism. A younger suite of granitoids occurs in the Tananao Metamorphic Complex of Taiwan and includes the Yuantoushan and Tachoshui granites (87.3 Ma) and the Kanagan gneiss (90.2 Ma). These mid-Cretaceous ages for the younger granitoid suite mitigate against the existence of the hypothetical late Paleozoic “Dongnanya” microcontinent in the study area as proposed by other investigators. UPb zircon inheritance ages, ranging between 1.67 and 2.09 Ga indicate the presence of a Precambrian basement beneath much of coastal SE China and Taiwan, however, there is no evidence of Archean crust as indicated by zircon inheritance patterns for rocks west of the present study area. On a87Sr86Sr vs.87Rb86Sr plot, the available data for the granitoids in Chinmen and coastal Fujian, SE China, however, define pseudoisochrons with apparent ages of 158–167 Ma. Although the pseudoisochron could be interpreted as a result of a two-end-member mixing process with subsequent fractional crystallization, the apparent age of the pseudoisochron, substantiated by the circumstantial evidence, is postulated as the approximate time of Sr-isotope homogenization of the crustal protoliths of the granitoids during the previous (early Yanshanian) tectonic event in this region. Such a Sr-isotope homogenization process must have occurred over a large area, possibly facilitated by a metamorphic fluid phase or by large-scale anatexis. The Cretaceous granitoids in this area were probably produced by partial melting of this crust in a more restricted scale. The Cretaceous calc-alkaline granitoids in coastal Fujian and Taiwan are of I-type or contaminated I-type. A subduction component is inferred during the magma genesis. Although the Taiwan granitoids are suggested to be subduction related, the possible tectonic environment for the genesis of coastal Fujian granitoids is yet to be solved.