摘要
桐柏山造山带南麓随州-枣阳地区的超镁铁-镁铁质岩墙(床)群分为性质完全不同的两类:橄长质岩墙(床)群和变质的基性岩墙群.其中橄长质岩墙(床)群未经历过任何变形和变质作用的影响,其侵位时代应不早于区内高压-超高压变质作用的时间(220～240 Ma),可能属晚造山或后造山岩浆作用的产物.这些橄长质岩墙(床)群中主要造岩矿物为橄榄石、斜长石、斜方辉石和单斜辉石,含少量金云母.其中橄榄石的Fo介于72～78之间,为贵橄榄石;斜长石以倍长石为主,少数为拉长石;单斜辉石主要为普通辉石,少数为透辉石;金云母中含较高的TiO2(3.36%),属钛金云母变种.橄榄石和普通辉石之间的平衡温度主要介于1 120～1 165℃之间,普通辉石的结晶温度为971～1 079℃.橄长质岩墙(床)群在地球化学上以高Al2O3(17.25%～20.20%)和MgO(9.56%～15.30%)含量、高的MgO/FeO*值(1.26～1.76)以及低TiO2(0.33%～0.58%)和低碱(Na2O+K2O=1.64%～2.23%)为特征,过渡元素的丰度(Cr=70～125μg/g、Ni=251～518μg/g、Co=54～75μg/g)和Mg#值(54～67)均较高,其性质可近似代表上地幔部分熔融所形成的母岩浆.稀土元素含量低(ΣREE=18.66～35.42μg/g),轻、重稀土元素之间的分馏程度较强,(La/Yb)N=4.4～4.7,尤以强的Ba、Sr及Eu正异常(δEu=1.31～1.49)为显著特征,表明其中有较多的富钙斜长石聚集.高场强元素Nb、Ta、U、Th以及Zr和Hf无明显异常,不同于区内早古生代玄武岩所揭示的富集型上地幔的地球化学特征,而与造山带内及前、后陆地区古近纪玄武岩的地球化学特征非常相似,可能表明造山带山根拆沉后,发生过软流圈地幔与大陆岩石圈地幔强烈的相互作用.橄长质岩墙群的出现说明俯冲板块的后缘曾发生过岩石圈范围的强烈拉张作用.
The ultrarnafic-mafic dyke swarms on the southern piedmont of the Tongbai orogenic belt in Suizhou-Zaoyang area consists of two remarkably different types： troctolitic dyke （sill） swarnzs and metamorphosed mafic dyke ones. The troctolitic dyke swarms did not experience any metamorphism, .so its emplacement age might be not earlier than that of the HP-UHP metamorphism （220- 240 Ma）, belonging probably to the product of late- to post-orogenic magmatism. Nevertheless, the metamorphosed mafic dyke swarms are geochemically very similar to Late Proterozoic basalt of Huashan Group in the area and are thus likely to be a cognate phase. The troctolitic dyke swarms consist chiefly of olivine, plagioclase, orthopyroxene and clinopyroxene with minor phlogopite. Olivine is chrysolite （Fo 72- 78）. Plagioclase crystals are mainly bytownite with minor labradorite. Clinopyroxene includes augite and diopside. The TiO2 content of phlogopite （3.36%） is high and indicates a Ti-rich variety. The olivine-augite equilibrium temperatures mainly range from 1 120 to 1 165℃, and the crystallization temperatures of augites range from 971 to 1 079℃. The rocks are characterized by high Al2O3 （ 17.25 % - 20.20 % ）, MgO （ 9.56 % - 15.30 % ） and MgO/FeO^＊ （ 1.26 - 1.76 ）, and low TiO2 （0.33% -0.58% ） and alkaline （Na2O＋ K2O= 1.64% -2.23% ）. The transitional element contents （Cr= 70-125μg/g, Ni= 251-518μg/g, Co= 54-75μg/g） and the Mg numbers （Mg^# = 54-67） are relatively high in the rocks, which may approximately represent partial melting of the parental magma in the upper mantle. The rare earth element contents of the troctolitic swarms are low （∑REE = 18.66-35.42μg/g） and are characterized by relative strong fractionation between LREE and HREE [（ La/Yb）N= 4.4-4.7] and especially by prominent features of strong Ba, Sr and Eu positive anomalies （Eu/Eu^＊ = 1.31 - 1.49）, thus suggesting relative enrichment of Ca-plagioclase in the swarms. There exist no HFS （high field strength） elements （Nb, Ta, U, Th, Zr, Hf） anomalies in the troctolitic dyke swarms. Such phenomena are quite different from geochemical characteristics of the enrichment-type upper mantle indicated by the early Paleozoic basalt in the area, but similar to the features of Paleogene basaltic rocks in the orogenic belt, the foreland and the backland. It is thus considered that, with the delamination of the mountain root, intense interaction occurred between the subcontinental lithospheric mantle and the lower mantle. The development of troctolitic dyke （sill） swarms means a strong extensional event in the back part of the subducted plate after the delamination of the mountain root.
出处
《岩石矿物学杂志》
CAS
CSCD
北大核心
2005年第5期409-417,共9页
Acta Petrologica et Mineralogica
基金
国家自然科学基金资助项目(40472035)
国家重点基础研究发展规划资助项目(2003CB716502)
