Compared with the major and trace elements of typical boninite, the metabasalts collected from the Nanfanba （南范坝）-Miaowanli （庙湾里） region in the Bikou （碧口） block could be treated as boninite characterized...Compared with the major and trace elements of typical boninite, the metabasalts collected from the Nanfanba （南范坝）-Miaowanli （庙湾里） region in the Bikou （碧口） block could be treated as boninite characterized by low-Si, low-Ti, low-P, high-Mg^2 and high Al2O3/TiO2, consistent with geochemical features of boninite. The normal mid-ocean ridge basalt （N-MORB） normalized spider diagram displays fairly depleted high field strong elements （HFSE） （Zr, Y, Ti）. Enriched refractory elements （Cr, Co, Ni） as well as light rare earth elements （LREE）-depleted chondrite-normalized REE distribution patterns suggest the boninitic magmas are derived from an extremely depleted mantle wedge in the presence of a hydrous fluid, meanwhile signifying the source region had previously undergone a high degree partial melting process yielding primary magmas with enriched large ion lithophile elements （LILE）. In addition, almost all the samples in the Nb-Zr-Y and Ti-Zr-Y discrimination diagrams were plotted in the island arc basalt （IAB） field. Coupled with the island arc tholeiitic （IAT） basalt in the study region, therefore, the geochemical characteristics of the studied rocks indicate the meta-basalts probably occurred in a fore-arc subduction setting. This conclusion may be of great significance for the further study of the tectonic background of the Bikou volcanism.展开更多
The Blkou Group on the Shaanxi-Gansu-Sichuan border is composed of Mid-Late Proterozoic metamorphosed bimodal volcanic rocks and flysch sediments. Its metamorphism may be divided into the blueschist and greenschist fa...The Blkou Group on the Shaanxi-Gansu-Sichuan border is composed of Mid-Late Proterozoic metamorphosed bimodal volcanic rocks and flysch sediments. Its metamorphism may be divided into the blueschist and greenschist facies. Three metamorphlc zones, i.e. zones A, B, and C, may be distinguished on the basis of the field distribution of metamorphlc rocks and the variation of b0 values of muscovite. Blueschists are characterized by coexistence of sodic amphiboles and epidote and occur as stripes or relict patches in extensive greenschists of zone A. Studies of metamorphic minerals such as amphiboles, chlorite, epidote and muscovite and their textural relationships indicate that blueschists and greenschists were not formed under the same metamorphic physico-chemical conditions. The blueschist facies was formed at temperatures of 300-400℃ and pressures of 0.5-0.6 GPa. The greenschist facies in zones A and B has similar temperatures but its pressure is only 0.4 GPa or so. The transition from the blueschist to greenschist facies is a nearly isothermal uplift process. The rock and mineral assemblages of the Bikou Group indicate that the blueschist facies metamorphism of the group might be related to crustal thickening or A-subduction accompanying the closure of an intracontinental small ocean basin.展开更多
YANGBA Formation of Bikou Group is located in south Qinling, a famous orogenic belt separating Chinesecontinent into two parts: North China plate and Yangzi plate. The Yangba Formation comprises the maficand felsic vo...YANGBA Formation of Bikou Group is located in south Qinling, a famous orogenic belt separating Chinesecontinent into two parts: North China plate and Yangzi plate. The Yangba Formation comprises the maficand felsic volcanic rocks formed in an ancient seafloor environment during the Proterozoic era. The suitesof mafic and felsic volcanic rocks are petrochemically divided into tholeiite and calc-alkaline dacite, respectively, showing characteristics of bimodal volcanic rock mainly by lithology and major element content, and a lack of typical intermediate rocks. These signatures indicate that the volcanic rocks of YangbaFormation were generated in rift tectonics setting. The tholeiites of Yangba Formation are similar in trace element content to mid-ridge basalt, displaying a slight depletion to enrichment in light rare earth elements (LREE’s) (La/Yb_n = 0.6-1.4), slightfractionation between LREE (La/Sm_n =0.55-1.14) and HREE (Gd/Lu_n = 1.09-1.50), and relative flat patterns in whole. The decoupling of Eu to its neighboring elements in two samples likely indicates an effect of fractional crystallization in magma evolution. The La-La/Sm, Ni-La, Ni-Th, Ni-Ta,and Ni-Hf diagrams all support a conclusion that the mafic rocks are generated by partial melting pro-展开更多
铧厂沟地区碧口群玄武岩呈带状和透镜状分布于凝灰岩和凝灰质千枚岩中。在详细野外观察基础上,利用显微镜观察、主微量元素及放射性同位素分析等综合分析技术对玄武岩的岩石学、地球化学以及年代学特征进行了研究。结果表明：碧口群玄...铧厂沟地区碧口群玄武岩呈带状和透镜状分布于凝灰岩和凝灰质千枚岩中。在详细野外观察基础上,利用显微镜观察、主微量元素及放射性同位素分析等综合分析技术对玄武岩的岩石学、地球化学以及年代学特征进行了研究。结果表明：碧口群玄武岩普遍经历了绿片岩相的变质作用,主要矿物为斜长石、绿泥石和绿帘石；SiO2（44.67%~49.76%）、TiO2（1.14%~1.34%）含量较低, TFe2O3（12.03%~15.47%）、MgO（7.57%~9.3%）、CaO（7.29%~10.54%）含量较高。稀土总量较低,轻稀土亏损,重稀土富集。岩石微量元素和锆石Hf同位素特征显示玄武岩形成环境可能类似N-MORB,起源于软流圈,并在上升过程中混入了古老的地壳物质。LA-ICP-MS锆石测年结果显示玄武岩的形成年龄为（316.3±6.0） Ma （MSWD=0.78；n=7）,表明碧口群火山岩系至少是两期（新元古代时期和晚古生代时期）火山作用的产物。其大规模形成时期为新元古代,后经晚古生代火山作用的叠加改造而成。展开更多
基金This paper is supported by the National Natural Science Foundation ofChina (No .40234041) .
文摘Compared with the major and trace elements of typical boninite, the metabasalts collected from the Nanfanba （南范坝）-Miaowanli （庙湾里） region in the Bikou （碧口） block could be treated as boninite characterized by low-Si, low-Ti, low-P, high-Mg^2 and high Al2O3/TiO2, consistent with geochemical features of boninite. The normal mid-ocean ridge basalt （N-MORB） normalized spider diagram displays fairly depleted high field strong elements （HFSE） （Zr, Y, Ti）. Enriched refractory elements （Cr, Co, Ni） as well as light rare earth elements （LREE）-depleted chondrite-normalized REE distribution patterns suggest the boninitic magmas are derived from an extremely depleted mantle wedge in the presence of a hydrous fluid, meanwhile signifying the source region had previously undergone a high degree partial melting process yielding primary magmas with enriched large ion lithophile elements （LILE）. In addition, almost all the samples in the Nb-Zr-Y and Ti-Zr-Y discrimination diagrams were plotted in the island arc basalt （IAB） field. Coupled with the island arc tholeiitic （IAT） basalt in the study region, therefore, the geochemical characteristics of the studied rocks indicate the meta-basalts probably occurred in a fore-arc subduction setting. This conclusion may be of great significance for the further study of the tectonic background of the Bikou volcanism.
基金This research is a project(No.49070098)supported by the National Natural Science Foundation of China
文摘The Blkou Group on the Shaanxi-Gansu-Sichuan border is composed of Mid-Late Proterozoic metamorphosed bimodal volcanic rocks and flysch sediments. Its metamorphism may be divided into the blueschist and greenschist facies. Three metamorphlc zones, i.e. zones A, B, and C, may be distinguished on the basis of the field distribution of metamorphlc rocks and the variation of b0 values of muscovite. Blueschists are characterized by coexistence of sodic amphiboles and epidote and occur as stripes or relict patches in extensive greenschists of zone A. Studies of metamorphic minerals such as amphiboles, chlorite, epidote and muscovite and their textural relationships indicate that blueschists and greenschists were not formed under the same metamorphic physico-chemical conditions. The blueschist facies was formed at temperatures of 300-400℃ and pressures of 0.5-0.6 GPa. The greenschist facies in zones A and B has similar temperatures but its pressure is only 0.4 GPa or so. The transition from the blueschist to greenschist facies is a nearly isothermal uplift process. The rock and mineral assemblages of the Bikou Group indicate that the blueschist facies metamorphism of the group might be related to crustal thickening or A-subduction accompanying the closure of an intracontinental small ocean basin.
文摘YANGBA Formation of Bikou Group is located in south Qinling, a famous orogenic belt separating Chinesecontinent into two parts: North China plate and Yangzi plate. The Yangba Formation comprises the maficand felsic volcanic rocks formed in an ancient seafloor environment during the Proterozoic era. The suitesof mafic and felsic volcanic rocks are petrochemically divided into tholeiite and calc-alkaline dacite, respectively, showing characteristics of bimodal volcanic rock mainly by lithology and major element content, and a lack of typical intermediate rocks. These signatures indicate that the volcanic rocks of YangbaFormation were generated in rift tectonics setting. The tholeiites of Yangba Formation are similar in trace element content to mid-ridge basalt, displaying a slight depletion to enrichment in light rare earth elements (LREE’s) (La/Yb_n = 0.6-1.4), slightfractionation between LREE (La/Sm_n =0.55-1.14) and HREE (Gd/Lu_n = 1.09-1.50), and relative flat patterns in whole. The decoupling of Eu to its neighboring elements in two samples likely indicates an effect of fractional crystallization in magma evolution. The La-La/Sm, Ni-La, Ni-Th, Ni-Ta,and Ni-Hf diagrams all support a conclusion that the mafic rocks are generated by partial melting pro-
文摘铧厂沟地区碧口群玄武岩呈带状和透镜状分布于凝灰岩和凝灰质千枚岩中。在详细野外观察基础上,利用显微镜观察、主微量元素及放射性同位素分析等综合分析技术对玄武岩的岩石学、地球化学以及年代学特征进行了研究。结果表明：碧口群玄武岩普遍经历了绿片岩相的变质作用,主要矿物为斜长石、绿泥石和绿帘石；SiO2（44.67%~49.76%）、TiO2（1.14%~1.34%）含量较低, TFe2O3（12.03%~15.47%）、MgO（7.57%~9.3%）、CaO（7.29%~10.54%）含量较高。稀土总量较低,轻稀土亏损,重稀土富集。岩石微量元素和锆石Hf同位素特征显示玄武岩形成环境可能类似N-MORB,起源于软流圈,并在上升过程中混入了古老的地壳物质。LA-ICP-MS锆石测年结果显示玄武岩的形成年龄为（316.3±6.0） Ma （MSWD=0.78；n=7）,表明碧口群火山岩系至少是两期（新元古代时期和晚古生代时期）火山作用的产物。其大规模形成时期为新元古代,后经晚古生代火山作用的叠加改造而成。
