Pseudosection modeling for the garnet amphibolite samples from the Western Dabie Mountains show they have experienced similar HP metamorphic evolution with that of the adjected eclogites.The common assemblage of
The most of high/ultrahigh-pressure(HP/UHP)terranes of the world are characterized by the occurrence of numerous pods,lenses or layered blocks of eclogite and amphibolites(e.g.O’Brien,1997;Elvevold and Gilotti,2000;Z...The most of high/ultrahigh-pressure(HP/UHP)terranes of the world are characterized by the occurrence of numerous pods,lenses or layered blocks of eclogite and amphibolites(e.g.O’Brien,1997;Elvevold and Gilotti,2000;Zhang et al.,2003;and references there in).Field and petrological features suggest that amphibolites should展开更多
The high-pressure （HP） eclogite in the western Dabie Mountain encloses numerous hornblendes, mostly barroisite. Opinions on the peak metamorphic P-T condition, PT path and mineral paragenesis of it are still in disp...The high-pressure （HP） eclogite in the western Dabie Mountain encloses numerous hornblendes, mostly barroisite. Opinions on the peak metamorphic P-T condition, PT path and mineral paragenesis of it are still in dispute. Generally, HP eclogite involves garnet, omphacite, hornblendes and quartz, with or without glaucophane, zoisite and phengite. The garnet has compositional zoning with XMg increase, XCa and XMn decrease from core to rim, which indicates a progressive metamorphism. The phase equilibria of the HP eclogite modeled by the P-T pseudosection method developed recently showed the following： （1） the growth zonation of garnet records a progressive metamorphic PT path from pre-peak condition of 1.9-2.1 GPa at 508~C-514~C to a peak one of 2.3-2.5 GPa at 528~C-531~C for the HP eclogite; （2） the peak mineral assemblage is garnet＋omphacite＋glaucophane＋quartz_＋phengite, likely paragenetic with lawsonite; （3） the extensive hornblendes derive mainly from glaucophane, partial omphacite and even a little garnet due to the decompression with some heating during the post-peak stage, mostly representing the conditions of about 1.4-1.6 GPa and 580~C-640~C, and their growth is favored by the dehydration of lawsonite into zoisite or epidote, but most of the garnet, omphacite or phengite in the HP eclogite still preserve their compositions at peak condition, and they are not obviously equilibrious with the hornblendes.展开更多
In the western Dabie Mountain area, the eclogites have similar compositions and tectonic environment, which could be contrastively researched. Except for the reservation of the early structural deformation inside and ...In the western Dabie Mountain area, the eclogites have similar compositions and tectonic environment, which could be contrastively researched. Except for the reservation of the early structural deformation inside and outside of the eclogite lens, there is no obvious difference between the characteristics of the foliation and lineation in the eclogite lens from the one in surrounding region. So this paper concludes that the eclogites or blueschists (high pressure metamorphic rocks, i. e. HPM) are basically situated in the original position. The eclogites are mostly superposed by the ductile shear zone and show the feature of structural displacement, but so far we have not discovered any large scale structural zone to uplift eclogite return. Based on the analyses of finite strain measure, petrofabric analysis and TEM image for some minerals such as quartz and garnet, we could efficiently know the deforming characteristics of the eclogite in the prophase and anaphase of the main deforming epoch, and finally determine the forming condition of eclogite according to the strain and the differential stress. This paper puts forward preliminary conclusion that some HPM rocks could be formed by the deep layer embedding and local stress concentration in the process of regional metamorphism.展开更多
The geophysical investigations with seismic and MT methods were carried out in the east Dabie Mountain area in 1997, producing detailed results about crustal structures with good compatibility between different geophy...The geophysical investigations with seismic and MT methods were carried out in the east Dabie Mountain area in 1997, producing detailed results about crustal structures with good compatibility between different geophysical methods. After integrated interpretation of both avail-able geophysical and geological data, the author compiles a crustal tectonic section across the east Dabie Mountain (Plate II), which provides much more structural details with improved reliabil-ity due to geophysical constraints applied to the deep structures. The east Dabie orogenic belt can be divided into 4 geologic units from north to south: the north Huaiyang, the north Dabie, the south Dabie and the Susong. The Mesozoic northward subduction of the Yangtze craton caused the Yangtze crust to insert into the middle and lower crust of the Susong high-pressure metamorphic zone, while the middle and lower crust below the north Huaiyang and the Hefei basin contains the basement of the Sino-Korean craton. The middle and lower crust of the south Dabie is rather dif-ferent from that of the north Dabie, showing that the north and the south Dabie had different evolu-tional trajectories and should not belong to a single tectonic unit. The current crustal pattern has resulted mainly from deformation caused by the post-collisional intracontinental subduction of both the Yangtze and Sino-Korean cratons before the late Jurassic, and deformation caused by later crustal extension including doming and unroofing around the north Dabie. It can be inferred that the suture zone of the Triassic collision between the Sino-Korea and the Yangtze was located along the Xiaotian-Mozitan fault zone, which contains a group of normal faults dipping north in the upper crust, but becomes steep thrusts of dipping south in the middle crust, accompanied by ex-tensive deformation between this fault zone and the Hefei basin. The middle crust below the north Huaiyang unit is connected to the basement of the Sino-Korean craton, showing the plate com-pressional convergence between the Yangtze and Sino-Korean cratons in the post-collisional stage. The influence of this convergent event reached as far as to Huainan, the northern boundary of the Hefei basin. As a clear reflection appears at 22s TWT in the stack reflection sections, the lithospheric thickness of the east Dabie is about 78 km. The newly obtained geophysical data in-dicate that the thickness of the east Dabie UHPM rock slices is no more than 8 km, therefore they do not present evidences to support the hypothesis involved in whole-plate exhumation of the UHPM terranes.展开更多
文摘Pseudosection modeling for the garnet amphibolite samples from the Western Dabie Mountains show they have experienced similar HP metamorphic evolution with that of the adjected eclogites.The common assemblage of
文摘The most of high/ultrahigh-pressure(HP/UHP)terranes of the world are characterized by the occurrence of numerous pods,lenses or layered blocks of eclogite and amphibolites(e.g.O’Brien,1997;Elvevold and Gilotti,2000;Zhang et al.,2003;and references there in).Field and petrological features suggest that amphibolites should
基金funded by the National Natural Science Foundation of China (No.40525006 and 40372032).
文摘The high-pressure （HP） eclogite in the western Dabie Mountain encloses numerous hornblendes, mostly barroisite. Opinions on the peak metamorphic P-T condition, PT path and mineral paragenesis of it are still in dispute. Generally, HP eclogite involves garnet, omphacite, hornblendes and quartz, with or without glaucophane, zoisite and phengite. The garnet has compositional zoning with XMg increase, XCa and XMn decrease from core to rim, which indicates a progressive metamorphism. The phase equilibria of the HP eclogite modeled by the P-T pseudosection method developed recently showed the following： （1） the growth zonation of garnet records a progressive metamorphic PT path from pre-peak condition of 1.9-2.1 GPa at 508~C-514~C to a peak one of 2.3-2.5 GPa at 528~C-531~C for the HP eclogite; （2） the peak mineral assemblage is garnet＋omphacite＋glaucophane＋quartz_＋phengite, likely paragenetic with lawsonite; （3） the extensive hornblendes derive mainly from glaucophane, partial omphacite and even a little garnet due to the decompression with some heating during the post-peak stage, mostly representing the conditions of about 1.4-1.6 GPa and 580~C-640~C, and their growth is favored by the dehydration of lawsonite into zoisite or epidote, but most of the garnet, omphacite or phengite in the HP eclogite still preserve their compositions at peak condition, and they are not obviously equilibrious with the hornblendes.
文摘In the western Dabie Mountain area, the eclogites have similar compositions and tectonic environment, which could be contrastively researched. Except for the reservation of the early structural deformation inside and outside of the eclogite lens, there is no obvious difference between the characteristics of the foliation and lineation in the eclogite lens from the one in surrounding region. So this paper concludes that the eclogites or blueschists (high pressure metamorphic rocks, i. e. HPM) are basically situated in the original position. The eclogites are mostly superposed by the ductile shear zone and show the feature of structural displacement, but so far we have not discovered any large scale structural zone to uplift eclogite return. Based on the analyses of finite strain measure, petrofabric analysis and TEM image for some minerals such as quartz and garnet, we could efficiently know the deforming characteristics of the eclogite in the prophase and anaphase of the main deforming epoch, and finally determine the forming condition of eclogite according to the strain and the differential stress. This paper puts forward preliminary conclusion that some HPM rocks could be formed by the deep layer embedding and local stress concentration in the process of regional metamorphism.
基金This project was supported by Basic Research Fund of the Chinese Ministry of Land and Resources (Grant Nos. 9501205 20001010203)
文摘The geophysical investigations with seismic and MT methods were carried out in the east Dabie Mountain area in 1997, producing detailed results about crustal structures with good compatibility between different geophysical methods. After integrated interpretation of both avail-able geophysical and geological data, the author compiles a crustal tectonic section across the east Dabie Mountain (Plate II), which provides much more structural details with improved reliabil-ity due to geophysical constraints applied to the deep structures. The east Dabie orogenic belt can be divided into 4 geologic units from north to south: the north Huaiyang, the north Dabie, the south Dabie and the Susong. The Mesozoic northward subduction of the Yangtze craton caused the Yangtze crust to insert into the middle and lower crust of the Susong high-pressure metamorphic zone, while the middle and lower crust below the north Huaiyang and the Hefei basin contains the basement of the Sino-Korean craton. The middle and lower crust of the south Dabie is rather dif-ferent from that of the north Dabie, showing that the north and the south Dabie had different evolu-tional trajectories and should not belong to a single tectonic unit. The current crustal pattern has resulted mainly from deformation caused by the post-collisional intracontinental subduction of both the Yangtze and Sino-Korean cratons before the late Jurassic, and deformation caused by later crustal extension including doming and unroofing around the north Dabie. It can be inferred that the suture zone of the Triassic collision between the Sino-Korea and the Yangtze was located along the Xiaotian-Mozitan fault zone, which contains a group of normal faults dipping north in the upper crust, but becomes steep thrusts of dipping south in the middle crust, accompanied by ex-tensive deformation between this fault zone and the Hefei basin. The middle crust below the north Huaiyang unit is connected to the basement of the Sino-Korean craton, showing the plate com-pressional convergence between the Yangtze and Sino-Korean cratons in the post-collisional stage. The influence of this convergent event reached as far as to Huainan, the northern boundary of the Hefei basin. As a clear reflection appears at 22s TWT in the stack reflection sections, the lithospheric thickness of the east Dabie is about 78 km. The newly obtained geophysical data in-dicate that the thickness of the east Dabie UHPM rock slices is no more than 8 km, therefore they do not present evidences to support the hypothesis involved in whole-plate exhumation of the UHPM terranes.
