It is believed that the albite-potash-feldspar-greenschists in the middle Proterozoic Langshan Group in Tanyaokou District are synsedimentary potassic spilite （or poenite） based on the following facts and features: ...It is believed that the albite-potash-feldspar-greenschists in the middle Proterozoic Langshan Group in Tanyaokou District are synsedimentary potassic spilite （or poenite） based on the following facts and features: （ ⅰ ） they occur in beds and look blackish （or apple） green and compact; （ ⅱ ） they bear blasto-glomeroporphyritic and blastocrystalloclastic crystals which consist of （altered） albite laths with hollow skeleton crystal texture in which there are slender dark strips and minor pyroxene, and blasto-amygdaloid filled with pyrite, chlorite, magnetite, etc.; （ ⅲ ） the results of the petrochemical diagrams show that they are orthometamorphites; （ Ⅳ ） Sm-Nd isotopic age of them is 1 824 Ma (model age (t<sub>DM</sub>) is 2 006—2 132 Ma).展开更多
The genesis of spilite-keratophyric volcanic rock assemblages being the important part of submarine volcanic rocks has long been in contention, the focus of which is the origin of sodium. Basing on the discoveries of ...The genesis of spilite-keratophyric volcanic rock assemblages being the important part of submarine volcanic rocks has long been in contention, the focus of which is the origin of sodium. Basing on the discoveries of albite with disorder-transition structure in spilite-keratophyre and the primary sodic magmatic inclusions in quartz phenocrysts展开更多
The Ordovician volcanic rocks in the Mayaxueshan area have been pervasivelyaltered or metamorphosed and contain abundant secondary minerals such as albite, chlorite, epidote,prehnite, pumpellyite, actinolite, titanite...The Ordovician volcanic rocks in the Mayaxueshan area have been pervasivelyaltered or metamorphosed and contain abundant secondary minerals such as albite, chlorite, epidote,prehnite, pumpellyite, actinolite, titanite, quartz, and/or calcite. They were denoted as spilitesor spilitic rocks in terms of their petrographic features and mineral assemblages. The metamorphicgrades of the volcanic rocks are equivalent to that of the intercalated metaclastic rocks. Thisindicates that both the spilitic volcanic rocks and metaclastic rocks in the Mayaxueshan area haveformed as a result of Caledonian regional metamorphism. We suggest that the previously denotedspilitic rocks or altered volcanic rocks should be re-denoted as metabasalts or metabasaltic rocks.The metamorphic grade of the volcanic rocks increases with their age: prehnite-pumpellyite faciesfor the upper part of the Middle Ordovician volcanic rocks, prehnite-pumpellyite to lowergreenschist facies for the lower part of the Middle Ordovician volcanic rocks, and lower greenschistfacies for the Lower Ordovician volcanic rocks. The P-T conditions are estimated as T = 240 - 290deg C and P = 1.5-4.5 kbar for the lower part of the Middle Ordovician rocks, and T = approx 300 degC for the Lower Ordovician rocks. The variations of mineral assemblages occurring at differentdomains of the volcanic rocks were controlled by the variations of the effective bulk composition inthose domains during metamorphism. The geochemical characteristics of Mg-Al chromite in theMayaxueshan volcanic rocks are consistent with an origin of island arc environment.展开更多
文摘It is believed that the albite-potash-feldspar-greenschists in the middle Proterozoic Langshan Group in Tanyaokou District are synsedimentary potassic spilite （or poenite） based on the following facts and features: （ ⅰ ） they occur in beds and look blackish （or apple） green and compact; （ ⅱ ） they bear blasto-glomeroporphyritic and blastocrystalloclastic crystals which consist of （altered） albite laths with hollow skeleton crystal texture in which there are slender dark strips and minor pyroxene, and blasto-amygdaloid filled with pyrite, chlorite, magnetite, etc.; （ ⅲ ） the results of the petrochemical diagrams show that they are orthometamorphites; （ Ⅳ ） Sm-Nd isotopic age of them is 1 824 Ma (model age (t<sub>DM</sub>) is 2 006—2 132 Ma).
基金Project supported by the National Natural Science Foundation of China
文摘The genesis of spilite-keratophyric volcanic rock assemblages being the important part of submarine volcanic rocks has long been in contention, the focus of which is the origin of sodium. Basing on the discoveries of albite with disorder-transition structure in spilite-keratophyre and the primary sodic magmatic inclusions in quartz phenocrysts
文摘The Ordovician volcanic rocks in the Mayaxueshan area have been pervasivelyaltered or metamorphosed and contain abundant secondary minerals such as albite, chlorite, epidote,prehnite, pumpellyite, actinolite, titanite, quartz, and/or calcite. They were denoted as spilitesor spilitic rocks in terms of their petrographic features and mineral assemblages. The metamorphicgrades of the volcanic rocks are equivalent to that of the intercalated metaclastic rocks. Thisindicates that both the spilitic volcanic rocks and metaclastic rocks in the Mayaxueshan area haveformed as a result of Caledonian regional metamorphism. We suggest that the previously denotedspilitic rocks or altered volcanic rocks should be re-denoted as metabasalts or metabasaltic rocks.The metamorphic grade of the volcanic rocks increases with their age: prehnite-pumpellyite faciesfor the upper part of the Middle Ordovician volcanic rocks, prehnite-pumpellyite to lowergreenschist facies for the lower part of the Middle Ordovician volcanic rocks, and lower greenschistfacies for the Lower Ordovician volcanic rocks. The P-T conditions are estimated as T = 240 - 290deg C and P = 1.5-4.5 kbar for the lower part of the Middle Ordovician rocks, and T = approx 300 degC for the Lower Ordovician rocks. The variations of mineral assemblages occurring at differentdomains of the volcanic rocks were controlled by the variations of the effective bulk composition inthose domains during metamorphism. The geochemical characteristics of Mg-Al chromite in theMayaxueshan volcanic rocks are consistent with an origin of island arc environment.
