海南岛印支-燕山期花岗岩年代学格架与成因

海南岛分布着大面积的印支-燕山期岩浆岩, 是华南岩浆活动的重要组成部分。近年来, 对这些花岗岩已开展了一系列工作, 但其年代学格架仍需完善, 成因类型及源区性质仍需限定, 区域构造机制仍存在争议。本文详细梳理了已发表的海南岛晚古生代-中生代花岗岩地球化学和年代学数据, 结合作者近年来的相关工作, 对海南岛花岗岩的年代学格架和岩石地球化学特征进行总结, 并对源岩、岩浆演化过程和地球动力学背景进行讨论。海南岛印支期(二叠纪-三叠纪)花岗岩(部分具有片麻状构造)整体以NE走向展布, 形成于278~225Ma, 峰期为240Ma。岩相学和地球化学研究表明, 此期岩浆活动形成了具有不同地球化学特征的S型、I型和A型花岗岩。其中, S型花岗岩(278~241Ma)与I型花岗岩(272~233Ma)均为高钾钙碱性花岗岩, 具有较低的全岩锆饱和温度(平均 < 750℃), 低于大规模角闪石分解引起的脱水熔融所需的温度, 可能是由底侵玄武质岩浆释放的水加入到下地壳诱发部分熔融的产物; A型花岗岩(257~225Ma)的全岩锆饱和温度较高(>800℃), 表明源区无流体加入或为已有花岗岩生成的去水源岩在高温条件下脱水熔融形成。此外, 本区还存在少量燕山期花岗岩, 包括燕山早期(侏罗纪)岩浆活动(如本研究识别出的161Ma高钾钙碱性I型花岗岩)以及集中在白垩纪的三期侏罗纪-白垩纪岩浆活动(按年龄可分为: ca.120Ma、ca.110~90Ma和ca.70Ma, 峰期为ca.100Ma)。地球化学数据表明燕山期花岗岩具有高钾钙碱性I型特征, 源区中有幔源年轻组分的加入。据此, 本文作者认为: (1)印支-燕山期大规模的花岗岩浆作用与古太平洋板块向华南大陆俯冲有关; (2)古太平洋板块向华南大陆的俯冲可能起始于早-中二叠世(ca. 280Ma), 结束于晚白垩世末期(ca. 70Ma); (3)自二叠纪起至白垩纪末期安第斯型活动大陆边缘结束, 海南岛保存了古太平洋俯冲相关的岩浆活动记录。

Geochronological framework and petrogenesis of Indosinian-Yanshannian granites in Hainan Island

In recent years, a series of work on granites have been conducted on the widespread Indosinian-Yanshannian granites in Hainan Island which constitute part of the magmatic rocks of South China. However, the geochronological framework, genetic type and regional tectonic setting of these granites still need to be constrained. We made a detailed analysis of relevant literature as well as our own study regarding the ages and geochemical properties of the Indosinian-Yanshannian granites in Hainan Island, and a further discussion on the magmatic source and tectonic background controlling them. In Hainan Island, the Indosinian granites are mainly composed of granodiorite and monzogranite, and part of them show gneissic structure. The summary of high precision geochronology data suggests that they emplaced during 278~225Ma, and the main age peak is ca.240Ma. The geochemical features indicate they can be divided into S-, I- and A-type granites. Both S-type (278~241Ma) and I-type (272~233Ma) granites are high-K calc-alkaline, and their whole-rock zircon saturation temperatures (average < 750℃) are lower than the dehydration melting of amphibolite, indicating that these two types of granites might be products of water-flux melting. In contrast, the high whole-rock zirconium saturation temperatures (>800℃) of A-type granites (257~225Ma) indicate they are generated from vapour-absent melting of the residue remaining in the lower crust after production of a previous granite. Furthermore, there are very limited Jurassic magmatic rocks developed in Hainan Island, i.e., the newly identified 161Ma granite is high-K calc-alkaline with I-type affinity; while the Cretaceous granites (mainly monzogranite and granodiorite) can be divided into three periods: ca.120Ma, ca.110~90Ma and ca.70Ma, with a peak at ca.100Ma. The geochemical evidence implies that these granites are I-type granites with the source involving juvernile mantle components. On the basis of the above information, we propose that: (1) the large volume Indosinian-Yanshannian magmatic rocks are developed under the subduction of the Paleo-Pacific plate; (2) the initiation of the Paleo-Pacific plate subduction started since Early Permian (ca.280Ma), and the Andean-type active continental margin of South China lasted to Late Cretaceous (ca.70Ma); (3) Hainan Island preserves the subduction-related Indosinian-Yanshannian magmatism under the setting of the Andean-type active continental margin of east-southeast Asia.