Mineralogical and petrogeochemical characteristics of ultramafic rocks from the metamorphic basement of the Jiaobei terrane

Abstract

The ultramafic rocks,including serpentinite,serpentinized spinel-orthopyroxene peridotite,spinel olivine pyroxenite and spinel-bearing amphibolite, were collected from the metamorphic basement of Jiaobei terrene. They record three-stage mineral assemblages: Opx1+ Ol1+ Spl1,Opx2+ Ol2+ Spl2+ Amp2 and Amp3+ Srp3+ Mag3+ Cal,and experienced high amphibolite facies and greenschist facies metamorphism. Amphibole porphyroblasts pervasively occur in each type rock,formed during high amphibolite and green-schist facies metamorphism. No deformation was observed in the serpentinite,indicating that serpentinization occurred in the late stage by metasomatism. Chemical compositions of the orthopyroxenites and high Mg O spinel peridotites are of characteristics of cratonic peridotite. REE rations of( La / Sm)N1. 11 ~ 1. 41,( La / Yb)N0. 91 ~ 1. 61 in amphibolite,and no HSFE anomaly in multielement pattern in the ultramafic rocks indicate that the ultramafic rocks are probably formed in a continental rift environment. Spinel compositions in the spinel peridotities have Mg#0. 72 ~ 0. 79,Cr#0. 06 ~ 0. 12 showing the metasomatic origin. Fo 88. 42 ~ 90. 50 in harzburgite,dispersive distribution of major elements of ultramafic rocks,big variation of ∑REE,the genetic relationships of diagrams of ultrmafic rocks and amphibolites from Si / Al-( Mg + Fe) / Al and Mg / Ti-Fe / Ti are all indicative of that the ultramafic rocks from the metamorphic basement of the Jiaobei terrane are probably a cumulate origin of a set of mantle-derived magma. High Mg O content( 16%~ 42%) in ultramafic rocks exhibit a characteristics of the mantle from Precambrian basement,and high water contents of ultramafic rocks reflect the complicated metamorphic processes from long-term geological evolution.

膠北地塊變質基底的蛇紋巖、蛇紋巖化尖晶石方輝橄欖巖、尖晶石橄欖斜方輝石巖、角閃石巖自形成以后,經歷了早期的高角閃巖相和晚期綠片巖相的變質作用,因而記錄了三個階段的礦物組合:早期的Opx1+Ol1+Spl1;中期的Opx2+Ol2+Spl2+Amp2和晚期的Amp3+Srp3+Mag3+Cal組合。超鎂鐵巖中普遍存在的角閃石,具有粒狀變晶結構特征,形成于角閃巖相和綠片巖相的變質作用過程。蛇紋石沒有受到應力作用的跡象,顯示其形成于晚期非擠壓環境的交代變質作用過程。尖晶石橄欖巖中斜方輝石的礦物化學以及尖晶石橄欖巖的高Mg O含量都表現出克拉通橄欖巖的性質。角閃石巖的稀土元素配分圖解的(La/Sm)N的比值1.11~1.41,(La/Yb)N的比值0.91~1.61,超鎂鐵巖多元素圖解顯示無高場強元素異常,都表明膠北地塊變質基底的超鎂鐵巖形成的構造背景或是裂谷環境。尖晶石礦物的Mg#0.72~0.79、Cr#0.06~0.12表明交代變質成因。方輝橄欖巖橄欖石的Fo值88.42~90.50、超鎂鐵質巖石全巖主元素的分散性、較大的∑REE含量變化以及Si/Al-(Mg+Fe)/Al和Mg/Ti-Fe/Ti主元素的比值圖解所表現出的超鎂鐵巖與角閃石巖具有成因聯系等,都表明膠北變質基底的超鎂鐵巖可能是具有堆晶成因的一套幔源巖漿系列。高MgO含量(16%~42%)表現出前寒武紀變質基底的橄欖巖地幔性質;高水含量反映出漫長地質歷史時期變質作用過程。