Geochemical characteristics and petrogenesis of Late Cretaceous hypersthene-bearing intrusive rocks in the Gangdese batholith, southern Tibet

Abstract

冈底斯岩基南缘自西向东，从楠木林到米林广泛出露一系列含暗色细粒包体的含紫苏辉石侵入岩。这一系列含紫苏辉石的侵入岩具有斜长石以及紫苏辉石的巨晶（>5mm），呈现出堆晶结构。锆石U-Pb定年表明，这一系列含紫苏辉石侵入岩的结晶年龄为97~77Ma，并不随侵位位置具有显著的经度上的变化。含紫苏辉石的基性岩具有高的Al₂O₃（17.3%~18.2%）含量，较高的MgO（3.9%~4.1%）含量，FeO^T含量在8.7%~9.0%之间；低的Cr（<14.8×10^-6）和Ni（<15.0×10^-6）含量，基本不具有Eu的异常，富集大离子亲石元素（LILE）和LREE，亏损高场强元素（HFSE）。含紫苏辉石的中-酸性岩具有高的Al₂O₃（14.9%~18.8%）含量，高的Mg^#值（>39.7）；变化较大的Cr（5.7×10^-6~260×10^-6）和Ni（10.2×10^-6~78.2×10^-6）含量，具有微弱-强烈的Eu的负异常，富集LILE和LREE，亏损HFSE。暗色细粒包体与含紫苏辉石基性岩相比具有相似的SiO₂含量，FeO^T（8.1%~9.0%）含量，稍高的MgO（4.7%~5.4%）含量，Al₂O₃（18.1%~19.4%）含量以及Mg^#值（51.0~52.6）；具有与含紫苏辉石基性岩相似的微量元素分布和稀土元素配分模式。这一系列含紫苏辉石的侵入岩具有较低的初始Sr同位素比值（⁸⁷Sr/⁸⁶Sr_（t）=0.7037~0.7044），较高并变化较大的ε_Nd（t）值（+3.7~+9.4）和ε_Hf（t）值（+9.9~+14.6）。这些特征共同说明，经流体+熔体交代的地幔楔中软流圈部分在俯冲流体存在的情况下发生部分熔融形成母岩浆，其母岩浆随后与俯冲板片熔体发生混合。在岩浆演化过程中经历了单斜辉石、斜方辉石以及斜长石的分离结晶并最终形成了冈底斯岩基南缘出露的含紫苏辉石侵入岩。暗色细粒包体可以代表母岩浆的早期堆晶，是岩浆淬火作用的产物。母岩浆中大量流体的存在，使其结晶顺序为单斜辉石-斜长石（紫苏辉石），随后的堆晶作用使得这一系列侵入岩得以赋存紫苏辉石。;A series of hypersthene-bearing intrusive rocks occur along the southern margin of the Gangdese batholith from Nanmulin to Milin. These rocks show cumulate texture with plagioclase megacrysts and hypersthene megacrysts (>5mm). Zircon U-Pb dating indicates they were formed at a period of 97~77Ma with no change along with the longitudinal variation of the location. According to geochemical study, the hypersthene-bearing intrusive rocks include two types, the hypersthene-bearing mafic rocks and the hypersthene-bearing dioritic-granitic rocks, respectively. The former are characterized with: (1) high Al₂O₃ (17.3%~18.2%), MgO (3.9%~4.1%) contents, with the content of FeO^T from 8.7% to 9.0%; (2) low Cr (<14.8×10^-6) and Ni (<15.0×10^-6) contents, with insignificant Eu anomaly, enrichment in LILE and LREE, depletion in HFSE. While the later are characterized with: (1) high Al₂O₃ (14.9%~18.8%) contents, Mg^# (>39.7); (2) variable Cr (5.7×10^-6~260×10^-6) and Ni (10.2×10^-6~78.2×10^-6) contents, with slight-significant negative Eu anomaly, enrichment in LILE and LREE, depletion in HFSE. Furthermore, compared the mafic ones, the mafic microgranular enclaves contain similar amounts of SiO₂ and FeO^T, slightly higher amounts of MgO (4.7%~5.4%) and Al₂O₃ (18.1%~19.4%), and higher Mg^# (51.0~52.6). The mafic microgranular enclaves show the similar distribution of the trace element and REE with the mafic ones. As for their isotopic compositions, the hypersthene-bearing intrusive rocks are characterized by low Sr (⁸⁷Sr/⁸⁶Sr<0.7044) but high ε_Nd(t) (>+3.7) and ε_Hf(t) (>+9.9) isotope compositions. These features suggest that the source region of their parental magmas should be the mantle wedge metasomatized by the fluid and melt derived from the subducting slab. The parental magmas suffered a mixing of melt of the metasomatized asthenosphere and melt of the subducting slab with various degrees of fractional crystallization of clinopyroxene, hypersthene and plagioclase in the process of magma evolution. The mafic microgranular enclaves in these intrusive rocks that represented the cumulation at an early stage should be generated by quenching process in the parental magmas system. The sequence of mineral fractional crystallization (clinopyroxene-plagioclase and hypersthene) in response to large-volume fluids in the parental magmas combined with subsequent cumulation makes this suit of intrusive rocks bearing hypersthene.