摘要
马拉山花岗岩位于特提斯喜马拉雅的西部,其主要矿物组成为石英、钾长石、白云母和黑云母。锆石LA-MC-ICP-MS U-Pb定年表明,花岗岩的发育记录了(28.0±0.5)Ma和(18.4±0.3)Ma两期深熔作用,(18.4±0.3)Ma代表了最终的结晶时间。全岩地球化学分析结果显示,样品具有高的SiO_2(72.36%~72.51%)、Al_2O_3(15.22%~15.37%)和CaO(1.64%~1.66%)含量,高的K_2O/Na_2O值(0.97-1.05)和A/CNK值(1.15-1.20),显示高钾钙碱性过铝质的特征;岩石富集Rb、Th、U和K,亏损Ba、Nb、Sr和Zr、Eu负异常不明显(δEu=0.80~0.89),轻重稀土分馏较强[(La/Yb)_N=7.09~19.68]。马拉山花岗岩具有较低的Rb/Sr值(0.90-1.10)和较高的CaO/Na_2O值(0.44~0.46),指示岩浆源区物质成分可能以页岩为主;样品(^(87)Sr/^(86)Sr)_i和ε_(Nd)(t)分别为0.742 522~0.744 097和-14.5^-13.7,与大喜马拉雅结晶杂岩中变质沉积岩成分一致,表明其来自变质沉积岩的部分熔融。岩石具有较低的(^(87)Sr/^(86)Sr);和较高的Sr含量,且随着Ba含量的增加,Rb/Sr值基本不变,表明马拉山花岗岩是水致白云母部分熔融的产物,部分熔融可能与南北向裂谷的东西向伸展关系密切。
The Malashan granite was exposed in the west of the Tethyan Himalayan sedimentary sequence(THS).It is characterized by gneissic texture and the assemblage of quartz,K-feldspar,muscovite and biotite.LA-MC-ICP-MS zircon U-Pb dating results indicated that the granite recorded two episodes of anatexis at(28.0 ±0.5) Ma and(18.4±0.3) Ma,respectively.The age of(18.4±0.3) Ma represents the final crystallized time of Malashan granite.Geochemical data show that these samples are characterized by high SiO2(72.36%-72.51%),A12O3(15.22%-15.37%),CaO(1.64%-l.66%) and high value of K2O/Na2O(0.97-1.05) and A/CNK(1.15-1.20),and the enrichment in Rb,Th,U and K,the depletion in Ba,Nb,Sr and Zr,weak negative Eu anomalies(δEu = 0.80-0.89),and strong fractionation between LREE and HREE((La/Yb)N= 7.09-19.68).These features suggest that they are high potassium calc-alkaline and peraluminous granites.The relatively low Rb/Sr ratios(0.90-1.10) and high CaO/Na2O ratios(0.44-0.46) imply that the magma source was probably psammitolite.The(87^Sr//86^Sr)1(0.742 522-0.744 097) and εNd(t)(- 14.5--13.7) can compare well with those of the metasedimentary rocks in the Greater Himalaya Crystalline complex(GHC),so these granite wasgenerated from partial melting of the GHC metasedimentary rocks.The features of relatively low(87^Sr/86^Sr,and high Sr content,and the constant Rb/Sr ratios relative to large variations in Ba concentrations approve that,the Malashangranite wasderived from fluxed melting of the GHC metasedimentary rocks,possibly associated tightly with the E-W extension along the North-South trend rift system(NSTR).
出处
《西北地质》
CAS
CSCD
2015年第4期70-87,共18页
Northwestern Geology
基金
中央级公益性科研院所基本科研业务专项(ZDJ2014-09)
国家自然科学基金"藏南佩枯错片麻岩穹窿变形-变质作用及形成演化研究"(41402175)联合资助