摘要
本文对太行山南段符山高镁闪长岩进行了年代学与地球化学研究,结合其中地幔橄榄岩包体的研究,对符山高镁闪长岩的成因和中生代岩石圈地幔的性质进行了探讨。研究表明,符山闪长岩体是由一套含橄榄石角闪闪长岩-角闪闪长岩-闪长岩构成。含橄榄岩包体的寄主岩——角闪闪长岩中的锆石可划分为两种:一是代表寄主岩浆结晶的锆石:内部结构均匀、呈条带状吸收、自形-半自形晶形,具有较高的Th/U比值(1.10～4.08),其^(206)Pb/^(238)U年龄介于123～128Ma之间,12个点的加权平均值为125±1Ma,这表明岩体的形成时代为早白垩世;二是捕获或继承锆石:具有核边结构、吸收程度不均匀、呈浑圆状和自形-半自形两种,它们的Th/U比值介于0.32～2.03之间,构成了3组^(207)Pb/^(206)Pb加权平均年龄:2503±11Ma、2181±26Ma和1778±36Ma。该类岩石的SiO_2和MgO含量分别介于56.69%～59.21%和3.60%～6.33%之间;且以高Mg~#(0.51～0.64)、富Na(Na_2O/K_2O大于1)、高Cr(93.1×10^(-6)～420×10^(-6))、Ni(35.1×10^(-6)～137×10^(-6))为特征。该类岩石强烈富集轻稀土元素和大离子亲石元素、明显亏损高场强元素,(^(87)Sr/^(86)Sr)_i、ε_(Nd)(t)值和(^(206)Pb/^(204)Pb)_i分别变化于0.70581～0.70641、-8.30～-16.56和17.052～17.512之间。综合上述特征,同时结合地慢橄榄岩包体的特征和古元古代捕获锆石的大量存在,认为符山高镁闪长岩的原始岩浆起源于拆沉陆壳物质的部分熔融,其后经历了与地幔橄榄岩的反应过程。
This paper reports chronological and geochemical data of the Fushan high-Mg diorites from the southern Taihang Mts. and discusses its petrogenesis and nature of the Mesozoic lithospheric mantle beneath the central North China Craton together with the mantle peridotite xenoliths entrained by them. The Fushan dioritic intrusion is composed mainly of olivine-bearing hornblende-diorite, hornblende-diorite and diorite. Based on their shape and structure, zircons from the hornblende-diorite hosting in peridotite xenoliths can be subdivided into two types, i. e. , one being the crystallizing ones from the host magma with zonal homogeneous absorption and euhedral-subhedral shape, the other being inherited or captured ones with core-rim structure, round and euhedral-subhedral shapes. The former has high Th/U ratios (1.10～4.08) , indicating a magmatic origin. Their ^(206)Pb/^(238)U ages range from 123Ma to 128Ma, yielding a mean ^(206)Pb/^(238)U age of 125±1 Ma (n = 12). This age represents the crystallization time of the Fushan high-Mg~# diorile. The latter' s Th/U ratios are between 0.32 and 2.03, and they yield three age populations of 2503±11 Ma, 2181±26Ma and 1778± 36Ma. SiO_2 and MgO contents of the Fushan dioritic rocks range from 56. 69% to 59. 21% and from 3.60% to 6.33% , respectively. Meanwhile, the dioritic rocks are characterized by high Mg~#(0.51～0.64) , enrichment in Na ( Na_2O/K_2O ratios more than 1 ) , Cr (93.1×10^(-6)～420×10^(-6)) and Ni (35.1×10^(-6)～137×10^(-6)) , and intensive enrichments in light rare earth elements (LREE) and large ion lithophile elements (LILEs) and depletion in high field strength elements (HFSEs). Their initial ^(87)Sr/^(86)Sr and ^(206)Pb/^(204)Pb ratios as well as ε_(Nd)(t) values range from 0. 70581 to 0. 70641 , from 17. 052 to 17. 512, and from -8. 30 to - 16. 56, respectively. Taken together, combined with the nature of the mantle peridotite xenoliths and the occurrence of a large amount of the Paleoproterozoic inherited or captured ziorcons in the host rocks, it is suggested that the primary magma for the Fushan high-Mg diorites should be derived from the partial melting of a delaminated lower continental crust and subsequently developed the melt-peridotite reaction.
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2009年第8期1947-1961,共15页
Acta Petrologica Sinica
基金
国家自然科学基金(90814003)
教育部高校博士学科点基金(200801830039)
973项目(2009CB825005)资助
