Granite is one of the most important components of the continental crust on our Earth; it thus has been an enduring studied subject in geology. According to present knowledge, granite shows a great deal of heterogenei...Granite is one of the most important components of the continental crust on our Earth; it thus has been an enduring studied subject in geology. According to present knowledge, granite shows a great deal of heterogeneity in terms of its texture,structure, mineral species and geochemical compositions at different scales from small dike to large batholith. However, the reasons for these variations are not well understood although numerous interpretations have been proposed. The key point of this debate is whether granitic magma can be effectively differentiated through fractional crystallization, and, if so, what kind of crystallization occurred during the magmatic evolution. Although granitic magma has high viscosity because of its elevated SiO2 content, we agree that fractional crystallization is effectively processed during its evolution based on the evidence from field investigation,mineral species and its chemical variations, and geochemical compositions. These data indicate that crystal settling by gravitation is not the only mechanism dominating granitic differentiation. On the contrary, flow segregation or dynamic sorting may be more important. Accordingly, granite can be divided into unfractionated, fractionated(including weakly fractionated and highly fractionated) and cumulated types, according to the differentiation degree. Highly fractionated granitic magmas are generally high in primary temperature or high with various volatiles during the later stage, which make the fractional crystallization much easier than the common granitic melts. In addition, effective magmatic differentiation can be also expected when the magma emplaced along a large scale of extensional structure. Highly fractionated granitic magma is easily contaminated by country rocks due to its relatively prolonged crystallization time. Thus, granites do not always reflect the characteristics of the source areas and the physical and chemical conditions of the primary magma. We proposed that highly fractionated granites are an important sig展开更多
It has been proposed that the North China Craton(NCC)was thinned up to a thickness of>100 km during the Phanerozoic,and underwent an associated craton destruction.Evidently,it is an important topic worthy of future...It has been proposed that the North China Craton(NCC)was thinned up to a thickness of>100 km during the Phanerozoic,and underwent an associated craton destruction.Evidently,it is an important topic worthy of future study to understanding the mechanism of cratonic destruction and its role played in the continental evolution.After synthesized the global cratons of India,Brazil,South Africa,Siberia,East Europe(Baltic)and North America,we found that lithospheric thinning is common in the cratonic evolution,but it is not always associated with craton destruction.Most cratons was thinned by thermal erosion of mantle plume or mantle upwelling,which,however,may not cause craton destruction.Based on the studies of the North American and North China Cratons,we suggest that oceanic subduction plays an important role in caton destruction.Fluids or melts released by dehydration of the subducted slabs metasomatize the mantle wedge above and trigger extensive partial melting.More importantly,the metasomatized mantle lost its original rigidity and make craton easier to be deformed and then to be destoyed.Therefore,we suggest that the widespread crust-derived granite and large-scale ductile deformation within the continental crust can be regarded as the petrological and structural indicators of craton destruction,respectively.展开更多
Two types of quartzofeldspathic inclusions hosted by omphacite and garnet were identified in the Sulu UHP eclogites. The first consists of albite, quartz, and various amounts of K-feldspar. In contrast, the second con...Two types of quartzofeldspathic inclusions hosted by omphacite and garnet were identified in the Sulu UHP eclogites. The first consists of albite, quartz, and various amounts of K-feldspar. In contrast, the second consists predominantly of K-feldspar and quartz without any albite. The presence of quart-zofeldspathic inclusions within the UHP mafic eclogites indicates that partial melting occurred in deeply subducted continental crust via mica dehydration melting reactions at an early stage of rapid exhumation. Such a melting event generated hydrous Na-K-Al-Si melts. These melts infiltrated into the mafic eclogite and were captured by recrystallizing garnet or omphacite, which together followed by dehydration and crystallization to form feldspar-bearing polyphase inclusions. Formation of silicate melts within the deeply subducted continental slab not only provides an excellent medium to transport both mobile (LILE) and immobile (HFSE) elements over a large distance, but also induces effective changes in the physical properties of the UHP slab. This process could be a major factor that enhances rapid exhumation of a deeply subducted continental slab.展开更多
Previous studies have shown that the Archean basement is widely distributed throughout the Yangtze craton. To date, however, Archean basement terrains have not been found, except for a few Kongling high-grade metamorp...Previous studies have shown that the Archean basement is widely distributed throughout the Yangtze craton. To date, however, Archean basement terrains have not been found, except for a few Kongling high-grade metamorphic terrains in the Huangling dome that have been confirmed to be of Archean age. To further understand the basement component and crustal evolution of the Yangtze craton, we carried out a petrological, geochronological and geochemical study of the Jinshan K-feldspar granite emplaced within the Yangpo Group, located in Huji Town, Zhongxiang City, Hubei Province. Results indicate that the zircon SHRIMP U-Pb age of the Jinshan granite is 2655±9 Ma, placing it within the middle Neoarchean. Chemically, this pluton yields abundant silica and alkalis and is depleted in Ca, Mg and Ti. Furthermore, it is enriched in Rb, Th, Ga, Y and Zr, depleted in Sr, Ba, Nb and Ta, and especially lacking in Eu. High ratios of FeO*/MgO (32.0 to 58.7) and 104 ×Ga/Al (3.19 to 3.41) were also found. The pluton exhibits characteristics typical of A-type granites with crustal source magmas. Moreover, the meta-sedimentary rock association of the Yangpo Group, into which the pluton intruded, clearly shows relatively stable depositional environments of a shallow shelf sequence. Therefore, before the middle Neoarchean, the Yangtze craton contained mature continental crust. This breakthrough discovery opens a new window on the study of the formation and evolution of the Yangtze craton basement.展开更多
The total magmatic output in modern arcs,where continental crust is now being formed, is believed to derive from melting of the mantle wedge and is largely basaltic. Globally averaged continental crust, however, has a...The total magmatic output in modern arcs,where continental crust is now being formed, is believed to derive from melting of the mantle wedge and is largely basaltic. Globally averaged continental crust, however, has an andesitic bulk composition and is hence too silicic to have been derived directly from the mantle. It is well known that one way this imbalance can be reconciled is if the parental basalt differentiates into a mafic garnet pyroxenitic residue/cumulate(‘‘arclogite'') and a complementary silicic melt, the former foundering or delaminating into the mantle due to its high densities and the latter remaining as the crust.Using the Sierra Nevada batholith in California as a case study, the composition of mature continental arc crust is shown in part to be the product of a cyclic process beginning with the growth of an arclogite layer followed by delamination of this layer and post-delamination basaltic underplating/recharge into what remains of the continental crust.A model is presented, wherein continuous arc magmatism and production of arclogites in continental arcs are periodically punctuated by a delamination event and an associated magmatic pulse every *10–30 My. The recycling flux of arclogites is estimated to be *5 %–20 % that of oceanic crust recycling by subduction. Delaminated arclogites have the necessary trace-element compositions to yield time-integrated isotopic compositions similar to those inferred toexist as reservoirs in the mantle. Because of their low melting temperatures, such pyroxenites may be preferentially melted, possibly forming a component of some hotspot magmas.展开更多
基金supported by the National Natural Science Foundation of China (Grant No. 41130313)by the Strategic Priority Research Program (B) of the Chinese Academy of Sciences (Grant No. XDB03010200)
文摘Granite is one of the most important components of the continental crust on our Earth; it thus has been an enduring studied subject in geology. According to present knowledge, granite shows a great deal of heterogeneity in terms of its texture,structure, mineral species and geochemical compositions at different scales from small dike to large batholith. However, the reasons for these variations are not well understood although numerous interpretations have been proposed. The key point of this debate is whether granitic magma can be effectively differentiated through fractional crystallization, and, if so, what kind of crystallization occurred during the magmatic evolution. Although granitic magma has high viscosity because of its elevated SiO2 content, we agree that fractional crystallization is effectively processed during its evolution based on the evidence from field investigation,mineral species and its chemical variations, and geochemical compositions. These data indicate that crystal settling by gravitation is not the only mechanism dominating granitic differentiation. On the contrary, flow segregation or dynamic sorting may be more important. Accordingly, granite can be divided into unfractionated, fractionated(including weakly fractionated and highly fractionated) and cumulated types, according to the differentiation degree. Highly fractionated granitic magmas are generally high in primary temperature or high with various volatiles during the later stage, which make the fractional crystallization much easier than the common granitic melts. In addition, effective magmatic differentiation can be also expected when the magma emplaced along a large scale of extensional structure. Highly fractionated granitic magma is easily contaminated by country rocks due to its relatively prolonged crystallization time. Thus, granites do not always reflect the characteristics of the source areas and the physical and chemical conditions of the primary magma. We proposed that highly fractionated granites are an important sig
基金supported by National Natural Science Foundation of China(Grant Nos.41130313 and 91014007)
文摘It has been proposed that the North China Craton(NCC)was thinned up to a thickness of>100 km during the Phanerozoic,and underwent an associated craton destruction.Evidently,it is an important topic worthy of future study to understanding the mechanism of cratonic destruction and its role played in the continental evolution.After synthesized the global cratons of India,Brazil,South Africa,Siberia,East Europe(Baltic)and North America,we found that lithospheric thinning is common in the cratonic evolution,but it is not always associated with craton destruction.Most cratons was thinned by thermal erosion of mantle plume or mantle upwelling,which,however,may not cause craton destruction.Based on the studies of the North American and North China Cratons,we suggest that oceanic subduction plays an important role in caton destruction.Fluids or melts released by dehydration of the subducted slabs metasomatize the mantle wedge above and trigger extensive partial melting.More importantly,the metasomatized mantle lost its original rigidity and make craton easier to be deformed and then to be destoyed.Therefore,we suggest that the widespread crust-derived granite and large-scale ductile deformation within the continental crust can be regarded as the petrological and structural indicators of craton destruction,respectively.
基金Supported by the National Natural Science Foundation of China (Grant No. 40673027)Outlay Research Fund of Chinese Academy of Geological Sciences (Grant No.20071120101125)
文摘Two types of quartzofeldspathic inclusions hosted by omphacite and garnet were identified in the Sulu UHP eclogites. The first consists of albite, quartz, and various amounts of K-feldspar. In contrast, the second consists predominantly of K-feldspar and quartz without any albite. The presence of quart-zofeldspathic inclusions within the UHP mafic eclogites indicates that partial melting occurred in deeply subducted continental crust via mica dehydration melting reactions at an early stage of rapid exhumation. Such a melting event generated hydrous Na-K-Al-Si melts. These melts infiltrated into the mafic eclogite and were captured by recrystallizing garnet or omphacite, which together followed by dehydration and crystallization to form feldspar-bearing polyphase inclusions. Formation of silicate melts within the deeply subducted continental slab not only provides an excellent medium to transport both mobile (LILE) and immobile (HFSE) elements over a large distance, but also induces effective changes in the physical properties of the UHP slab. This process could be a major factor that enhances rapid exhumation of a deeply subducted continental slab.
基金supported by the National Natural Science Foundation of China (41030315 and 41072088)the Chinese Geological Survey (1212011121111 and 1212011220750)
文摘Previous studies have shown that the Archean basement is widely distributed throughout the Yangtze craton. To date, however, Archean basement terrains have not been found, except for a few Kongling high-grade metamorphic terrains in the Huangling dome that have been confirmed to be of Archean age. To further understand the basement component and crustal evolution of the Yangtze craton, we carried out a petrological, geochronological and geochemical study of the Jinshan K-feldspar granite emplaced within the Yangpo Group, located in Huji Town, Zhongxiang City, Hubei Province. Results indicate that the zircon SHRIMP U-Pb age of the Jinshan granite is 2655±9 Ma, placing it within the middle Neoarchean. Chemically, this pluton yields abundant silica and alkalis and is depleted in Ca, Mg and Ti. Furthermore, it is enriched in Rb, Th, Ga, Y and Zr, depleted in Sr, Ba, Nb and Ta, and especially lacking in Eu. High ratios of FeO*/MgO (32.0 to 58.7) and 104 ×Ga/Al (3.19 to 3.41) were also found. The pluton exhibits characteristics typical of A-type granites with crustal source magmas. Moreover, the meta-sedimentary rock association of the Yangpo Group, into which the pluton intruded, clearly shows relatively stable depositional environments of a shallow shelf sequence. Therefore, before the middle Neoarchean, the Yangtze craton contained mature continental crust. This breakthrough discovery opens a new window on the study of the formation and evolution of the Yangtze craton basement.
文摘The total magmatic output in modern arcs,where continental crust is now being formed, is believed to derive from melting of the mantle wedge and is largely basaltic. Globally averaged continental crust, however, has an andesitic bulk composition and is hence too silicic to have been derived directly from the mantle. It is well known that one way this imbalance can be reconciled is if the parental basalt differentiates into a mafic garnet pyroxenitic residue/cumulate(‘‘arclogite'') and a complementary silicic melt, the former foundering or delaminating into the mantle due to its high densities and the latter remaining as the crust.Using the Sierra Nevada batholith in California as a case study, the composition of mature continental arc crust is shown in part to be the product of a cyclic process beginning with the growth of an arclogite layer followed by delamination of this layer and post-delamination basaltic underplating/recharge into what remains of the continental crust.A model is presented, wherein continuous arc magmatism and production of arclogites in continental arcs are periodically punctuated by a delamination event and an associated magmatic pulse every *10–30 My. The recycling flux of arclogites is estimated to be *5 %–20 % that of oceanic crust recycling by subduction. Delaminated arclogites have the necessary trace-element compositions to yield time-integrated isotopic compositions similar to those inferred toexist as reservoirs in the mantle. Because of their low melting temperatures, such pyroxenites may be preferentially melted, possibly forming a component of some hotspot magmas.
