冈底斯晚第三纪斑岩的岩石学、地球化学及其地球动力学意义

Neogene porphyries from Gangdese: petrological, geochemical characteristics and geodynamic significances.

西藏高原冈底斯中新世(21～12 Ma)侵位于冈底斯花岗岩岩基内.斑岩呈东西向断续带状展布(350 km),南北向串珠排布(宽约50 km),明显受NS裂谷和正断层控制.斑岩的主要岩石类型为花岗闪长斑岩、二长花岗斑岩、石英二长斑岩.斑岩高SiO2、Al2O3、K2O和低Y和HREE(Yb)含量;富集轻稀土元素(LREE)和大离子亲石元素(LILE), 尤其富集Sr和K; 缺乏Eu异常和/或具弱正异常;普遍具有较高的Sr/Y、(La/Yb)N比值及较低的Y/Yb比值.斑岩显示出明显的adakite地球化学特征,明显不同于冈底斯花岗岩基.对冈底斯岩带的构造-岩浆-热事件序列的综合分析揭示,斑岩是滞留在地幔的洋壳残片在花岗岩基大幅快速隆升后部分熔融的产物,熔融残余相为石榴石+角闪石+单斜辉石+含钛矿物组合,熔体在上升过程中与上覆交代地幔楔发生作用,使岩浆的K含量和Mg#明显增高.高密度的熔融残留体榴辉岩或角闪榴辉岩沉入下伏地幔或软流圈,这可能导致了冈底斯岩带在20～14 Ma期间的隆升.冈底斯中新世斑岩的时空分布厘定了西藏高原冈底斯带EW向伸展的初始时间大约在20Ma左右.

The distribution of Neogene felsic porphyries located in earlier granitoid batholiths was mainly controlled by NS-striking rifting zones and normal faults. The felsic porphyries were spatially and temporally allocated with the basaltic dike swarms. The main rock types of the porphyries are granodiorite-porphyry, monzonitic granite-porphyry and quartz monzonitic porphyry. The porphyries are characterized by high-SiO2, Al2O3, K2O, Sr and Mg-# and low CaO, Y and HREE (Yb) contents with high Sr/Y and La/Yb ratios, and show shoshonitic or high-K calc-alkaline magmatic affinity. The porphyries have enrichment pattern of LILE and LREE with lack of Eu anomalies, stronger negative HFSE spikes as Nb and Ti. The porphyries show characteristics of adakite. Trace element systems indicate that the source of the porphyries is characterized by the presence of garnet, amphibole, clinopyroxene, and perhaps some titanate phase. On the base of the sequence of observable structural-thermal-magmatic events at Gangdese, Neogene porphyries in Gangdese were formed by partial melting of dead subducted slab during post-collision. K-enrichment in the porphyries is attributed to the interaction of slab-derived melts, i.e., adakites, with the metasomatized mantle during ascent. There might be a delamination of residual phase with eclogite/amphibole-eclogite. The delamination caused latest uplift during 20 similar to 14Ma in Gangdese. The temporal and spatial distribution of the felsic porphyries and basaltic dike swarms indicates that the initial age of EW extension in Gangdese is around 20Ma.