【作者】 蔡宏明；

【导师】 张宏飞；

【作者基本信息】 中国地质大学 ， 地球化学， 2010， 博士

【摘要】 松潘-甘孜褶皱带(简称松潘带)位于青藏高原的东北缘,是中国大陆最大的构造结,覆盖面积大于200,000 km2。北部以阿尼玛卿-勉略缝合带为界,与东昆仑-西秦岭造山带相接；东部以龙门山断裂为界,与扬子块体相接；西南部以金沙江缝合带为界,与青藏高原的羌塘-昌都块体相接。松潘带内充填有巨厚层(5～15km)的三叠系复理石沉积,除东南部丹巴地区出露的震旦纪-古生界地层外,无前寒武纪基底暴露。由于古特提斯洋的封闭及其后华北块体、扬子块体及羌塘-昌都块体的汇聚作用,松潘带强烈变形。松潘带内广泛分布印支期花岗岩类,有少量火山岩出露。前人对松潘带东部及东南部的I型花岗岩、A型花岗岩、强过铝质S型花岗岩和埃达克质花岗岩类进行了详细的研究,这些花岗岩类主要来自于地壳物质的部分熔融。尽管软流圈地幔的上涌被用来解释地壳熔融的热机制,但是来自幔源的物质贡献报道少见。先前提出的松潘带印支期岩石圈拆沉作用模型尚缺少该区火山岩类及幔源岩浆研究的支持。本文对松潘带西部巴颜喀拉地区和中部达日地区花岗岩类、久治地区安山岩进行了岩相学、LA-ICPMS锆石U-Pb年代学、主量元素、微量元素及Sr-Nd同位素及锆石Hf同位素的综合研究,重点讨论了研究区印支期岩浆岩的成因,构造背景。结合前人研究成果,阐明松潘带印支期深部作用过程。获得的主要成果如下：(1)获得了松潘带西部及中部地区主要岩浆岩LA-ICPMS锆石U-Pb年代学数据,建立了松潘带印支期岩浆岩年代学格架。松潘带西部巴颜喀拉地区扎朵、巴颜喀拉岩体的年龄分别为219±2 Ma和216±5 Ma,这一年龄范围与松潘带东部埃达克质岩浆的形成年龄基本重合,说明巴颜喀拉地区花岗岩类和东部埃达克质岩浆可能在同一构造背景下形成。中部达日地区桑日麻、建设、莫坝岩体的年龄分别为208±1Ma,205±2Ma,206±2Ma;久治地区阿坝和洼赛安山岩的年龄分别为210±3 Ma,205±1 Ma。结合前人研究成果认为,松潘带印支期岩浆岩活动以中酸性侵入岩为主,主要发生在243～185 Ma,峰期出现在208 Ma,可划分为243～228 Ma,221～216 Ma,211～200 Ma三个期次,第一个期次岩浆岩活动较弱,主要分布在东南部地区；第二个期次岩浆岩主要以钙碱性壳源侵入岩为主,与加厚地壳的熔融有关；第三个期次岩浆岩出现碱性花岗岩类和火山岩类,与岩石圈的伸展减薄有关；(2)对松潘带中部达日地区的碱性花岗岩(桑日麻碱性正长岩)进行了研究。该岩体具有A1型花岗岩的特征。以中等的SiO2含量(56.92～61.80%),较高的全碱(K2O+Na2O=9.23～12.26%、TFeO/MgO(10.88～31.98)、Rb (254-400 ppm)、HFSE (Nb=61.23-133.74 ppm.Ta=3.87-8.18 ppm.Zr=505.9-1379.6 ppm、Hf=8.98-23.79 ppm、Y=37.08-102.29 ppm)和较低的MgO含量(0.15～0.81%)为特征；同时桑日麻岩体富集轻稀土,轻稀土分馏较重稀土更加明显,具有明显负Eu异常,呈“V”字形；该岩体ISr值为0.70723～0.70806,εNd(t)值为-0.3～0.0,εHf(t)=+4.9～+11.1。近于0的εNd(t)值和较高的εHf(t)值说明桑日麻正长岩中含有亏损地幔物质。该岩体可能由岩石圈地幔来源的岩浆经过分异结晶形成,岩浆演化过程中有亏损地幔物质的贡献,地壳物质的混染作用有限。桑日麻岩体的产出可能反映了后碰撞环境向板内伸展环境的过渡。在松潘带印支期花岗岩类中,桑日麻岩体具有最高的锆石饱和温度,此时软流圈上升达到最大高度,岩石圈伸展达到最大规模；(3)首次对松潘带印支期火山岩类进行了研究。松潘带中部久治地区阿坝和洼赛火山岩主要是钙碱性-高钾钙碱性安山岩。阿坝安山岩的岩浆源区主要来自下部地壳物质的部分熔融,含有地幔物质的参与,其岩浆源区位置可能在壳幔边界,而洼赛火山岩的岩浆源区应来自于岩石圈地幔物质的部分熔融。这一地幔源区受到一定程度的地幔流体交代作用；(4)揭示了松潘带西部基底性质。松潘带西部巴颜喀拉地区扎朵和扎牙石英闪长岩,巴颜喀拉、珍秦及岗音巴拉花岗闪长岩主要是钙碱性-高钾钙碱性I型花岗岩类。地球化学及Sr-Nd-Hf同位素结果表明它们可能是角闪石脱水熔融诱发镁铁质下地壳部分熔融形成。岩浆形成过程中可能有富集地幔的加入和中地壳物质的混染。野外地质,年代学及微量元素特征表明它们形成在后碰撞构造背景。这些花岗岩类的源区性质指示了松潘带西部具有未出露的大陆基底,该基底在地球化学组成上类似于松潘带东部,具有扬子板块的属性；(5)讨论了松潘带印支期深部作用过程。松潘带印支期岩浆岩包括钙碱性安山岩和多种成因类型的花岗岩类,如强过铝质S型花岗岩、I型花岗岩类、埃达克质花岗岩类、钾玄质花岗岩、A型花岗岩类,源区涉及到中地壳、下地壳、壳幔过渡带、岩石圈地幔和软流圈物质。大陆碰撞后,加厚岩石圈的拆沉模式可以较好的解释这些岩浆岩的成因,不同源区来源的岩浆岩反映了不同深度物质在上涌软流圈热作用下的部分熔融。幔源碱性花岗岩和火山岩的岩石成因为松潘带印支期岩石圈发生拆沉作用提供又一新的证据,表明松潘带印支期岩石圈拆沉作用导致残留岩石圈地幔部分熔融。(6)探讨了松潘带印支期构造演化过程。古特提斯洋封闭之后,由于华北板块、扬子板块和羌塘块体的汇聚,松潘带缩短增厚；晚三叠世早期加厚岩石圈拆沉,软流圈地幔上涌,使加厚下地壳发生熔融；晚三叠世中晚期,软流圈物质进一步上涌,松潘带进入板内伸展构造体制,岩浆作用涉及到岩石圈地幔甚至软流圈地幔的部分熔融。更多还原

【Abstract】 The Songpan-Garze fold belt, located at the northeastern margin of the Tibetan Plateau, is the largest tectonic jounction in China. It covers a huge triangular area (>200,000 km2) confined by the North China, the Yangtze and the Qiangtang blocks. To the north, the limit is marked by the A’nimaque-Mianlue suture zone. To the east the Longmenshan thrust-nappe belt separates the belt from the Yangtze block. To the southwest, the Songpan-Garze fold belt is bounded by the Jinshajiang suture zone, which is considered as a Late Paleozoic Paleo-Tethys oceanic subduction zone dipping either to the west or the east. The belt is mainly filled by the Triassic flysch sediments with a thickenss of 5-15 km, which had experienced strong folding deformation during the Indosinian compressional tectonism due to closure of the Paleo-Tethys ocean and subsequent convergence between the North China, the Yangtze and the Qiangtang continental blocks. No pre-Cambrian basement is exposed within the Songpan-Garze fold belt, except for the Sinian crystallization basement exposed in Danba area, southeastern part of the belt.Within the Songpan-Garze fold belt, granitoids are widespread and volcanic rocks are rare, both can provide important information on geodynamics. Previous studies demonstrated that those granitoids from eastern and southeastern belt have variety genetic types, such as adakitic, A-type, I-type and strongly peraluminous granitoids, and mainly derived form ancient continental crust. It is infered that the crust partial melting process was triggered by the flux of heat coming from the rising asthenospheric material. However, the magma directly from mantle (including lithospheric mantle and asthenospheric mantle) has been reported rarely in the belt. In the Songpan-Garze fold belt, an Indosinian lithospheric delamination model has been proposed based on previous investigation on widespread granitoids. This model lacks information from volcanic magmatism and mantle-derived magmas. During the Indosinian delamination in the Songpan-Garze fold belt, whether partial melting of lithospheric mantle taken place is noteworthy.This paper reports petrography, U-Pb zircon LA-ICP-MS ages, geochemical and Sr-Nd isotopic and zircon Hf isotopic compositions of the granitoids from the Bayankala area of the western belt and Dari area of the central belt, as well as volcanic rocks from Jiuzhi area of the central belt. We use these data to discuss their magma source and petrogenesis. Our study provides constraints on Late Triassic deep geologic process of the belt. The main research results are as follows:(1) The precise chronology data by LA-ICPMS zircon U-Pb method for the magmatic rocks from the western and central Songpan-Garze fold belt are obtained. Results show that the Zhaduo and Bayankala plutons from the Bayankala area in the western belt have magmatic crystallization ages of 219±2 Ma and 216±5 Ma, respectively. They are consistent with each other within error, and imply that these plutons were likely product of a common magma event. The magmatism of western belt mainly occurred during 216-219 Ma, synchronous with the massive emplacement of adakitic granitic plutons from eastern belt, implying that they have had the same geodynamic setting. The Sangrima, Jianshe and Moba plutons from Dari area have magmatic crystallization ages of 208±1Ma,205±2 Ma and 206±2 Ma, respectively. And the Aba and Waisai andesites have magmatic crystallization ages of 210±3 Ma and 205±1 Ma, respectively.Combined with previous geochronology results, it is indicated that the Indosinian magmatism in the Songpan-Garze fold belt mainly occurred during 243-185 Ma, with a highest peak at～208 Ma. And the magmatisms are divided into three stages:the first one is the 243-228 Ma weak magma activity; the second one is the 221-216 Ma thickened crust-derived calc-alkaline intrusives; the third one is 211-200 Ma magmas characterized by presence of mantle derived alkaline rocks, corresponding to lithosphere extension.(2) The Sangrima pluton is mainly alkline syenite, belonging to A1-type granitoid. They have intermediate SiO2 (56.92-61.80%), high K2O+Na2O (9.23-12.26%), TFeO/MgO (10.88-31.98), Rb (254-400 ppm), HFSE (Nb=61.23-133.74 ppm, Ta=3.87-8.18 ppm, Zr= 505.9-1379.6 ppm, Hf=8.98-23.79 ppm, Y=37.08-102.29 ppm) and low MgO (0.15-0.81%) contents. The REE compositions show moderately fractionated patterns with (La/Yb)N=7.1-8.8 and Eu/Eu*= 0.28-0.53. Sr-Nd-Hf isotopic data for the syenite show ISr=0.70723-0.70806,εNd(t) =-0.3-0.0,εHf(t)=+4.9-+11.1. The near-zeroεNd (t) and highεHf(t) values indicate involvement of a depleted mantle source. The syenites were formed by crystallization fractionation of lithospheric mantle-derived melt with involvement of depleted mantle and limited crustal assimilation. The occurrence of the Sangrima A1-type granitoid is probably related to environment transfer from post-collisional to intraplate extension. The syenites have highest zircon saturation temperatures among the Songpan-Garze granitoids, probably corresponding to maximum asthenospheric material and maximum continental extension.(3) Obtain the first detail geochemical report on volcanic rocks of Songpan-Garze fold belt. The Aba and Wasai volcanic rocks from Jiuzhi area are calc-alkaline andesites. Geochemical and Sr-Nd-Hf isotopic compositions indicate that the magma of the Aba andesites was dominantly originated from a crustal source, with minor mantle-derived component. The magma generation location is likely at the boundary between crust and mantle. The magma of the Wasai andesites resulted from partial melting of lithosphere mantle, which was probably metasomatized by amphibole-bearing fluid.更多还原