滇西腾冲地块高黎贡山群早志留世变质花岗岩体的年代学、地球化学特征及意义

Chronological and geochemical characteristics of the Early Silurian metamorphic granites in Tengchong Block,western Yunnan and their implications

西南特提斯构造带广泛发育的早古生代岩浆岩是冈瓦纳大陆边缘原特提斯洋增生造山作用的产物,目前报导的岩浆岩侵位时代在536~448Ma。本文通过LA-ICP-MS锆石U-Pb定年,在腾冲地块东缘高黎贡山群中首次发现了年轻至~437Ma的片麻状花岗质岩体,并结合其锆石Hf同位素和全岩主微量地球化学特征,进一步制约原特提斯洋构造演化过程。样品主量元素显示此片麻状花岗岩体具有高硅(SiO_2=72.78%~73.69%)、富碱(K_2O+Na_2O=7.23%~8.70%)的过铝质(A/CNK=1.08~1.12)特征,微量元素显示此岩体相对富集轻稀土元素、大离子亲石元素(K、Rb)和Pb,亏损高场强元素(Nb、Ta、P、Zr、Ti)以及Ba、Sr、Eu。综合岩石样品的矿物组合特征和地球化学特征,判断该岩体为S型花岗岩,源于以砂屑岩为主的沉积岩类的部分熔融,且源区有斜长石的残留。锆石εHf(t)值(-9.8^-6.2)和二阶段模式年龄tDM2(2.0~1.8Ga)也表明其源于古老地壳沉积物,且无幔源物质加入。根据全岩锆饱和温度计和锆石Ti温度计得出其岩浆从源区发生部分熔融到固结的过程中,温度从794℃左右下降到约754℃。熔浆温度较高,推测源区部分熔融过程中有地幔热的供给。综合前人研究成果,冈瓦纳大陆边缘在早古生代依次经历了原特提斯洋板片俯冲(ca.530~510Ma)、地块群增生与洋板片断离(ca.510~490Ma)、岩石圈挤压增厚(ca.490~475Ma)和岩石圈地幔拆沉(ca.470~460Ma)。岩石圈地幔拆沉将导致软流圈上涌及随后大陆岩石圈的持续伸展。腾冲地块侵位于~437Ma的花岗质岩体系该拆沉构造后的伸展环境中,以砂屑岩为主的古老地壳沉积岩在地幔热的供给下发生部分熔融的产物。

The Early Paleozoic magmatic rocks,which are ubiquitous in the southwestern Tethys tectonic belt,are the product of the proto-Tethys accretionary orogenesis in the Gondwana continent. The current geochronological studies reported these plutons formed in the range of 536 - 448 Ma. In this paper,a granitic outcrop formed in - 437 Ma is first discovered in Gaoligongshan Group through LAICP-MS zircon U-Pb dating. The zircon Hf-isotope and bulk-rock major and trace element data of the granites are reported to further constrain the proto-Tethyan tectonic evolution. The major element data reveal these Early Silurian granites are high silicic（ Si O2=72. 78% - 73. 69%）,high alkali（ K2 O ＋ Na2 O = 7. 23% - 8. 70%） and peraluminous（ A/CNK = 1. 08 - 1. 12）. The trace element data exhibit they are relatively enriched in LREEs,LILEs（ Rb,K） and Pb,and depleted in HFSEs（ Nb,Ta,P,Zr,and Ti）,Ba,Sr and Eu. All the mineral assemblage characteristics and geochemical features of the rock simples are comparable to those of S-type granites attributed to partial melting of metasediments dominated by psammite with residual plagioclase in the source area. ZirconεHf（ t）（-9. 8 --6. 2） and tDM2（ 2. 0 - 1. 8Ga） also prove those S-type granites are mainly derived from the ancient crustal metasedimentary,with little mantle-derived components introduced into the melt. According to zircon saturation temperatures（ TZr）and Ti-in-zircon thermometer,the temperature decreases from - 794℃ to - 754℃ during the process of partial melting to magma consolidation. The high temperature of the melt indicates the mantle may as a source for the extra heat required for crustal anataxis.Previous research indicates in the Early Paleozoic Gondwana continental margin experienced proto-Tethyan subduction（ ca. 530 -510Ma）,accretionary orogenesis of microcontinental fragments and slab breakoff（ ca. 510 - 490Ma）,lithospheric thickening（ ca. 490- 475Ma）,and lithospheric delamination（ ca. 475 - 460Ma）. The demolition of the lithospheric mantle will lead to asthenosphere upwelling and continued extension of continental lithosphere. Under the tectonic extension,the granitic rocks emplaced in - 437 Ma in Tengchong Block may be produced through partial melting of the ancient crustal metasediments dominated by psammite,and mantle may supply some extra heat for the partial melting process.