There are several Pan-African granitoid plutons widely distributed in Shillong Plateau,NE India.Nongpoh(506.7±7 Ma)and Mylliem(480-430 Ma)plutons were chosen for the petrological and geochemical study to constrai...There are several Pan-African granitoid plutons widely distributed in Shillong Plateau,NE India.Nongpoh(506.7±7 Ma)and Mylliem(480-430 Ma)plutons were chosen for the petrological and geochemical study to constrain their petrogenesis.Nongpoh pluton consists of coarse-grained porphyritic quartz-monzogranite(NQM)and Mylliem pluton consists of medium to coarse-grained porphyritic granite(MG).The constituent minerals are K-feldspar,plagioclase,quartz,and biotite in both granitoids with accessory minerals of hornblende,zircon,sphene,and ilmenite.Both NQM and MG are metaluminous to weakly peraluminous(A/CNK=0.82-1.06),exhibit varied ranges of SiO2(NQM:58.4-64.9 wt%;MG:66.9-69.9 wt%),and display a clear compositional gap in the Harker variation diagram.NQM contains higher abundances of CaO,MgO and Fe2O3t and similar K2O and total alkali contents compared to MG.They have distinctive geochemical features typical of highly fractionated I-type granitoids such as higher abundances of K2O,Al2O3,MgO,CaO,Al2O3+CaO>15 wt%and A/CNK<1.1,low P2O5 content,enrichment in LILE,depletion in HFSE and HREE and highly fractionated REE patterns with moderate Eu anomalies,implying magma generation in a post-collisional extension setting and thinning induced asthenosphere upwelling,accompanied by the partial melting of the overlying enriched lithospheric mantle.The multi-element diagram of both NQM and MG shows pronounced negative anomalies at Ba,Nb,Sr,P,Zr,and Ti which implies a major role of crystal fractionation in their petrogenesis.High concentrations of Th,U,and Pb in the granitoid types point to additional involvement of crustal components in their generation.However,MGs with more pronounced positive spikes at U,Th,and Pb compared to NQM in multi-element diagram suggests the involvement of more felsic crustal material.The observed geochemical features of the granitoid types thus suggest that they are genetically unrelated to each other and their parental magmas were modified during magmatic differentiation processes.We suggest that the NQM and MG were formed as a result of fractional crystallization of compositionally diverse hybrid magmas produced due to mingling and mixing of an enriched lithospheric mantle-derived melts with lower crust-and middle crust-derived melts respectively during a period of extension late in the cycle of PanAfrican orogeny.展开更多
The Baleigong granites, located in the western part of the southwestern Tianshan Orogen(Kokshanyan region, China), records late Paleozoic magmatism during the late stages of convergence between the Tarim Block and the...The Baleigong granites, located in the western part of the southwestern Tianshan Orogen(Kokshanyan region, China), records late Paleozoic magmatism during the late stages of convergence between the Tarim Block and the Central Tianshan Arc Terrane. We performed a detailed geochronological and geochemical study of the Baleigong granites to better constrain the nature of collisional processes in the Southwest Tianshan Orogen. The LA-ICP-MS U-Pb zircon isotopic analyses indicate that magmatism commenced in the early Permian(~282 Ma). The granite samples, which are characterized by high contents of SiO2(67.68-69.77 wt%) and Al2O3(13.93-14.76 wt%), are alkali-rich and Mg-poor, corresponding to the high-K calc-alkaline series. The aluminum saturation index(A/CNK) ranges from 0.93 to 1.02, indicating a metaluminous to slightly peraluminous composition. Trace element geochemistry shows depletions in Nb, Ta, and Ti, a moderately negative Eu anomaly(δEu=0.40-0.56), enrichment in LREE, and depletion in HREE((La/Yb)N=7.46-11.78). These geochemical signatures are characteristic of an I-type granite generated from partial melting of a magmatic arc. The I-type nature of the Baleigong granites is also supported by the main mafic minerals being Fe-rich calcic hornblende and biotite. We suggest that the high-K, calc-alkaline I-type granitic magmatism was generated by partial melting of the continental crust, possibly triggered by underplating by basaltic magma. These conditions were likely achieved in a collisional tectonic setting, thus supporting the suggestion that closure of the South Tianshan Ocean was completed prior to the Permian and was followed(in the late Paleozoic) by collision between the Tarim Block and the Central Tianshan Arc Terrane.展开更多
The Mabeng granitic intrusion, which occurs in the northwestern T-S gold ore field, presents intruive contact with the country rocks. The facies zoining in it is obvious and its mineral components are relatively simpl...The Mabeng granitic intrusion, which occurs in the northwestern T-S gold ore field, presents intruive contact with the country rocks. The facies zoining in it is obvious and its mineral components are relatively simple. Its petrochemic characteristics demonstrate that it is hornblende-biotite granite with aluminium-alkali oversaturation, meanwhile its petrogeochemical features show a high gold content with a concentrational coefficient 12. 2. Its REE suggests that its internal different facies are the differentiational products of magma from the same source. All of those support the suggestion that the intrusion is originaly magmatic genesis and belongs io the I-type of granite. The original magma of the body comes from the deep crust or the upper mantle and is highly auriferous.展开更多
基金financially supported by the Department of Science and Technology,Govt.of India under the Inspire Research Fellowship Programme(File No.:DST/Inspire Fellowship/2016/IF160812)。
文摘There are several Pan-African granitoid plutons widely distributed in Shillong Plateau,NE India.Nongpoh(506.7±7 Ma)and Mylliem(480-430 Ma)plutons were chosen for the petrological and geochemical study to constrain their petrogenesis.Nongpoh pluton consists of coarse-grained porphyritic quartz-monzogranite(NQM)and Mylliem pluton consists of medium to coarse-grained porphyritic granite(MG).The constituent minerals are K-feldspar,plagioclase,quartz,and biotite in both granitoids with accessory minerals of hornblende,zircon,sphene,and ilmenite.Both NQM and MG are metaluminous to weakly peraluminous(A/CNK=0.82-1.06),exhibit varied ranges of SiO2(NQM:58.4-64.9 wt%;MG:66.9-69.9 wt%),and display a clear compositional gap in the Harker variation diagram.NQM contains higher abundances of CaO,MgO and Fe2O3t and similar K2O and total alkali contents compared to MG.They have distinctive geochemical features typical of highly fractionated I-type granitoids such as higher abundances of K2O,Al2O3,MgO,CaO,Al2O3+CaO>15 wt%and A/CNK<1.1,low P2O5 content,enrichment in LILE,depletion in HFSE and HREE and highly fractionated REE patterns with moderate Eu anomalies,implying magma generation in a post-collisional extension setting and thinning induced asthenosphere upwelling,accompanied by the partial melting of the overlying enriched lithospheric mantle.The multi-element diagram of both NQM and MG shows pronounced negative anomalies at Ba,Nb,Sr,P,Zr,and Ti which implies a major role of crystal fractionation in their petrogenesis.High concentrations of Th,U,and Pb in the granitoid types point to additional involvement of crustal components in their generation.However,MGs with more pronounced positive spikes at U,Th,and Pb compared to NQM in multi-element diagram suggests the involvement of more felsic crustal material.The observed geochemical features of the granitoid types thus suggest that they are genetically unrelated to each other and their parental magmas were modified during magmatic differentiation processes.We suggest that the NQM and MG were formed as a result of fractional crystallization of compositionally diverse hybrid magmas produced due to mingling and mixing of an enriched lithospheric mantle-derived melts with lower crust-and middle crust-derived melts respectively during a period of extension late in the cycle of PanAfrican orogeny.
基金financially supported by the National Natural Science Foundation of China (Grant Nos.U1403292, 41472196, 41502085, and 41902214)the National Key Technology Research and Development Program of the Ministry of Science and Technology of China (2015BAB05B04, 2018YFC0604005)the China Geological Survey Bureau (JYYWF20183702, JYYWF20180602)
文摘The Baleigong granites, located in the western part of the southwestern Tianshan Orogen(Kokshanyan region, China), records late Paleozoic magmatism during the late stages of convergence between the Tarim Block and the Central Tianshan Arc Terrane. We performed a detailed geochronological and geochemical study of the Baleigong granites to better constrain the nature of collisional processes in the Southwest Tianshan Orogen. The LA-ICP-MS U-Pb zircon isotopic analyses indicate that magmatism commenced in the early Permian(~282 Ma). The granite samples, which are characterized by high contents of SiO2(67.68-69.77 wt%) and Al2O3(13.93-14.76 wt%), are alkali-rich and Mg-poor, corresponding to the high-K calc-alkaline series. The aluminum saturation index(A/CNK) ranges from 0.93 to 1.02, indicating a metaluminous to slightly peraluminous composition. Trace element geochemistry shows depletions in Nb, Ta, and Ti, a moderately negative Eu anomaly(δEu=0.40-0.56), enrichment in LREE, and depletion in HREE((La/Yb)N=7.46-11.78). These geochemical signatures are characteristic of an I-type granite generated from partial melting of a magmatic arc. The I-type nature of the Baleigong granites is also supported by the main mafic minerals being Fe-rich calcic hornblende and biotite. We suggest that the high-K, calc-alkaline I-type granitic magmatism was generated by partial melting of the continental crust, possibly triggered by underplating by basaltic magma. These conditions were likely achieved in a collisional tectonic setting, thus supporting the suggestion that closure of the South Tianshan Ocean was completed prior to the Permian and was followed(in the late Paleozoic) by collision between the Tarim Block and the Central Tianshan Arc Terrane.
文摘The Mabeng granitic intrusion, which occurs in the northwestern T-S gold ore field, presents intruive contact with the country rocks. The facies zoining in it is obvious and its mineral components are relatively simple. Its petrochemic characteristics demonstrate that it is hornblende-biotite granite with aluminium-alkali oversaturation, meanwhile its petrogeochemical features show a high gold content with a concentrational coefficient 12. 2. Its REE suggests that its internal different facies are the differentiational products of magma from the same source. All of those support the suggestion that the intrusion is originaly magmatic genesis and belongs io the I-type of granite. The original magma of the body comes from the deep crust or the upper mantle and is highly auriferous.
