华南地块东部早古生代的陆内造山作用：来自I型花岗岩的启示

摘要

华南地块东部的早古生代造山带是特殊的陆内造山带(主要分布于安化-罗城断裂和政和-大埔断裂之间),以广泛的强烈变质、变形作用和大量分布的花岗质岩为特征。华南地块东部地区的早古生代花岗岩以过铝质S型花岗岩为主, I型花岗岩分布少且研究程度相对较低。目前,对于华南大面积花岗岩的成因一直存在较大争议,是否有地幔物质的参与也并不明确。对扬子板块东南缘板杉铺和宏夏桥两个I型花岗闪长岩岩体的锆石U-Pb定年结果显示,板杉铺和宏夏桥岩体分别形成于432.4±2.6 Ma 和434.3±2.7 Ma。系统的地球化学分析显示,板杉铺岩体富 K2O(2.71%~3.46%)和Al2O3(15.17%~15.59%),具有高 Sr(344.8×10-6~466.6×10-6)、Sr/Y 比值(34~47)和低 Y(8.51×10-6~10.72×10-6),具有埃达克质岩石特征。其低εNd(t)(-7.5~-6.9)和高的87Sr/86Sri(0.7130~0.7169),以及较老的tDM2=1729~1776 Ma,暗示其来源于加厚的下地壳中古元古代物质的部分熔融。相比板杉铺岩体,宏夏桥岩体具有较低的 SiO2(64.40%~65.42%)含量,较高的TiO2(0.58%~0.63%)、Fe2O3(4.48%~4.79%)、CaO(3.34%~4.39%)和 MgO(2.55%~3.06%),以及较高的 Y 含量(14.56×10-6~17.82×10-6)和较低的 Sr/Y(20~24)比值。但其与板杉铺岩体相似的 Sr-Nd 同位素特征(87Sr/86Sri=0.7142~0.7149,εNd(t)=-7.7~-7.3),说明宏夏桥岩体具有与板杉铺岩体相似的物质源区。由于高钾钙碱性I型花岗质岩的形成需要有外部热源提供才能使其源区发生部分熔融,因而该地区陆内造山峰期(444 Ma)之后的垮塌所导致的高温软流圈上涌,可能为这些花岗质岩石的形成提供了热源。但是这些岩体富集的 Sr-Nd 同位素组成显示,软流圈来源的地幔物质并没有直接参与花岗质岩的形成。结合前人的研究成果,造山后伸展环境下的软流圈上涌可能是形成华南特殊的早古生代陆内造山带的一种重要机制,是形成华南有别于经典陆内造山带的大量早古生代花岗岩的主要原因。%The early Paleozoic intracontinental orogenic belt in the eastern South China Block is characterized by not only intensive deformation and metamorphism, but also extensive granitic magmatism. Granites in the eastern South China Block are predominately peraluminous S-type granites, and I-type granites are rare. Uncertainties remain for the petrogenesis of the I-type granitic intrusions in the area, especially the role of the mantle. Two representative granodioritic plutons in southeastern Yangtze Block, namely, the Banshanpu and Hongxiaqiao plutons were selected for LA-ICP-MS zircon dating and geochemistry analysis. The results indicated that the Banshanpu and Hongxiaqiao plutons were emplaced at 432.4±2.6 Ma and 434.3±2.7 Ma respectively, right on the peak time (434 Ma) of the early Paleozoic magmatism in South China Block. The Banshanpu pluton has high K2O (2.71%~3.46 %) and Al2O3 (15.17%~15.59%) contents with high Sr/Y(34~47) ratios, similar to adakitic rocks. The low εNd(t) (-7.5~-6.9) values, high 87Sr/86Sri (0.7130~0.7169) ratio and old tDM2 (1729~1776 Ma) indicate an origination of thickened Paleoproterozoic lower crust. In contrasting, the Hongxiaqiao pluton has relatively low SiO2 (64.40%~65.42%) content, higher TiO2 (0.58%~0.63%), Fe2O3(4.48%~4.79%), CaO (3.34%~4.39%), MgO (2.55%~3.06%) and Y (14.56×10-6~17.82×10-6) contents and lower Sr/Y (20~24) ratios. Sr-Nd isotopes (87Sr/86Sri=0.7142~0.7149,εNd(t)=-7.7~-7.3) of the Hongxiaqiao pluton are similar to those of the Banshanpu pluton, suggesting a similar source(s). Unlike S-type granites, petrogenesis of I-type granite requires extra heat to fuse the lower crust source. The post-orogenic collapse after intracontinental peak-metamorphism (444 Ma) led to the upwelling of the asthenosphere, which provided enough heat for partial melting of the source rocks, but the asthenosphere did not provide material for the granitic rocks. It is most likely that the upwelling of asthenosphere created the early Paleozoic intracontinental orogenic belt in Southern China Block.