2005 Vol. 30, No. 6
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2005, 30(6): 639-645.
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Abstract:
The high precision measurement of isotopic ratios for micro-geological samples has become very important in the geological and environmental sciences. Recently developed solid-source thermal ionization mass spectrometers are highly precise and sensitive, and are playing a crucial role in isotopic geochronology and geochemistry. This paper reports analytical data obtained from standard material solutions using an IsoProbe-T mass spectrometer. Measurements on the Sr standard material NBS987 and Nd standard material Ames yielded mean 87Sr/86Sr and 143Nd/144Nd ratio values of 0.710 241 8± 0.000 005 1 and 0.512 148 4±0.000 002 9, respectively, with an internal precision better than 0.000 3%. Measurements using a small amount of the Sr standard material solution NBS987 (0.3-1 ng) yielded an internal precision significantly better than 0.003%. Combining chemical separation procedures having low blank, measurements of micro-geological samples have successively been achieved. This combination of low-blank chemical procedures and the use of a high precision mass spectrometer will potently promote applications of single-grain geochronology and isotopic tracer on magmatic rocks, metamorphic rocks, ore deposits and tectonites.
The high precision measurement of isotopic ratios for micro-geological samples has become very important in the geological and environmental sciences. Recently developed solid-source thermal ionization mass spectrometers are highly precise and sensitive, and are playing a crucial role in isotopic geochronology and geochemistry. This paper reports analytical data obtained from standard material solutions using an IsoProbe-T mass spectrometer. Measurements on the Sr standard material NBS987 and Nd standard material Ames yielded mean 87Sr/86Sr and 143Nd/144Nd ratio values of 0.710 241 8± 0.000 005 1 and 0.512 148 4±0.000 002 9, respectively, with an internal precision better than 0.000 3%. Measurements using a small amount of the Sr standard material solution NBS987 (0.3-1 ng) yielded an internal precision significantly better than 0.003%. Combining chemical separation procedures having low blank, measurements of micro-geological samples have successively been achieved. This combination of low-blank chemical procedures and the use of a high precision mass spectrometer will potently promote applications of single-grain geochronology and isotopic tracer on magmatic rocks, metamorphic rocks, ore deposits and tectonites.
2005, 30(6): 646-658.
Abstract:
Mafic igneous rock, a widely distributed and typical mantle-derived rock, has become a major research source for revealing possible mechanisms for the chemical evolution of the mantle. Combined geological and geochemical studies have been carried out on the Mesozoic and Cenozoic mafic igneous rocks in western Shandong Province, determining the elemental and isotopic compositions of typical samples. Generally, the Mesozoic mafic igneous rocks have enriched LREE and LILE concentrations, low HFSE abundance, and display highly variable ISr ratios (0.703 96-0.712 47) and distinctly negative εNd (t) values (-9.20 to -21.21). Compared to the north of western Shandong Province, the Mesozoic mafic rocks in the south have higher total REE contents (ΣREE=325.52×10-6-555.75×10-6), higher LREE/HREE ratios (17.75-25.97), and higher LILE/HFSE ratios (e. g., La/Nb=6.37-13.85, Th/Nb=0.52-1.53). They also have more radiogenic Sr isotopic compositions, with ISr values of 0.708 44-0.712 47 and 0.703 96-0.705 98, respectively. Integrated elemental and isotopic tracing suggest that the Mesozoic mantle of western Shandong Province is generally characterized by the EMⅠ component, which was probably formed by large-scale lithospheric delamination, whereas the mantle source in the south part has been superimposed by the influence of the deep subducted Yangtze continental materials, and thus displays the mixed features of EMⅠ and EMⅡ components. The geochemical characteristics of the Cenozoic basalts are similar to those of the oceanic basalts. It is likely that the rocks were derived from a depleted asthenosphere source but underwent metasomatism shortly before partial melting. From the Mesozoic to the Cenozoic, the nature of the mantle beneath the North China craton evolved from an enriched one to a depleted one. This chemical evolution most likely resulted from an asthenosphere upwelling, which was induced by large-scale lithospheric delamination and thus replaced the original lithospheric mantle with a newly created one.
Mafic igneous rock, a widely distributed and typical mantle-derived rock, has become a major research source for revealing possible mechanisms for the chemical evolution of the mantle. Combined geological and geochemical studies have been carried out on the Mesozoic and Cenozoic mafic igneous rocks in western Shandong Province, determining the elemental and isotopic compositions of typical samples. Generally, the Mesozoic mafic igneous rocks have enriched LREE and LILE concentrations, low HFSE abundance, and display highly variable ISr ratios (0.703 96-0.712 47) and distinctly negative εNd (t) values (-9.20 to -21.21). Compared to the north of western Shandong Province, the Mesozoic mafic rocks in the south have higher total REE contents (ΣREE=325.52×10-6-555.75×10-6), higher LREE/HREE ratios (17.75-25.97), and higher LILE/HFSE ratios (e. g., La/Nb=6.37-13.85, Th/Nb=0.52-1.53). They also have more radiogenic Sr isotopic compositions, with ISr values of 0.708 44-0.712 47 and 0.703 96-0.705 98, respectively. Integrated elemental and isotopic tracing suggest that the Mesozoic mantle of western Shandong Province is generally characterized by the EMⅠ component, which was probably formed by large-scale lithospheric delamination, whereas the mantle source in the south part has been superimposed by the influence of the deep subducted Yangtze continental materials, and thus displays the mixed features of EMⅠ and EMⅡ components. The geochemical characteristics of the Cenozoic basalts are similar to those of the oceanic basalts. It is likely that the rocks were derived from a depleted asthenosphere source but underwent metasomatism shortly before partial melting. From the Mesozoic to the Cenozoic, the nature of the mantle beneath the North China craton evolved from an enriched one to a depleted one. This chemical evolution most likely resulted from an asthenosphere upwelling, which was induced by large-scale lithospheric delamination and thus replaced the original lithospheric mantle with a newly created one.
2005, 30(6): 659-672.
Abstract:
SHRIMP zircon U-Pb dating and oxygen isotope analysis were carried out for granites at Xinkailing in the Beihuaiyang zone. In cathodoluminescence (CL) images, most zircons in one granite sample exhibit primary oscillatory zoning, which is typical for magmatic zircon, but some grains have internal structure altered by hydrothermal fluid; whereas the oscillatory zonations of most zircons in the other granite have been significantly disturbed, reflecting much stronger modification by hydrothermal alteration. The zircon U-Pb dating for the oscillatory zoned and hydrothermally altered domains yields two groups of weighted mean 206Pb/238U ages at (820±4) Ma and (780±4) Ma, respectively, which are interpreted as timing of granite emplacement and hydrothermal alteration. According to the obtained ages, the formation and later supersolidus hydrothermal alteration of the Xinkailing granite correspond to the two stages of bimodal magmatism with ages of ca. 830 to 795 Ma and ca. 780 to 745 Ma, respectively, for pre-rift and syn-rift during the breakup of the supercontinent Rodinia. They have very low δ18O values, with 1.90‰ to 5.78‰ for zircon, -2.88‰ to -7.67‰ for quartz, and -4.01‰ to -11.40‰ for plagioclase. Zircon and other minerals display obvious O isotope disequilibrium, while the other minerals approached O isotope re-equilibration at different temperatures during the hydrothermal alteration. Zircons with a more strongly altered internal structure have lower relative δ18O values. Therefore, the high-T hydrothermal alteration at Xinkailing may have occurred under supersolidus conditions, which not only completely reset the O isotope composition of quartz, but also partly modified the O isotope composition of zircon. The low δ18O values of zircon in some samples may be partly acquired from the low δ18O hydrothermal fluid. The anomalously low δ18O values of -2.88‰ to -7.67‰ for quartz indicate that the hydrothermal fluid may be derived from the meteoric water of a cold paleoclimate. Therefore, the formation of low δ18O granite during the mid-Neoproterozoic at Xinkailing is correlated with rifting magmatism and the snowball earth event.
SHRIMP zircon U-Pb dating and oxygen isotope analysis were carried out for granites at Xinkailing in the Beihuaiyang zone. In cathodoluminescence (CL) images, most zircons in one granite sample exhibit primary oscillatory zoning, which is typical for magmatic zircon, but some grains have internal structure altered by hydrothermal fluid; whereas the oscillatory zonations of most zircons in the other granite have been significantly disturbed, reflecting much stronger modification by hydrothermal alteration. The zircon U-Pb dating for the oscillatory zoned and hydrothermally altered domains yields two groups of weighted mean 206Pb/238U ages at (820±4) Ma and (780±4) Ma, respectively, which are interpreted as timing of granite emplacement and hydrothermal alteration. According to the obtained ages, the formation and later supersolidus hydrothermal alteration of the Xinkailing granite correspond to the two stages of bimodal magmatism with ages of ca. 830 to 795 Ma and ca. 780 to 745 Ma, respectively, for pre-rift and syn-rift during the breakup of the supercontinent Rodinia. They have very low δ18O values, with 1.90‰ to 5.78‰ for zircon, -2.88‰ to -7.67‰ for quartz, and -4.01‰ to -11.40‰ for plagioclase. Zircon and other minerals display obvious O isotope disequilibrium, while the other minerals approached O isotope re-equilibration at different temperatures during the hydrothermal alteration. Zircons with a more strongly altered internal structure have lower relative δ18O values. Therefore, the high-T hydrothermal alteration at Xinkailing may have occurred under supersolidus conditions, which not only completely reset the O isotope composition of quartz, but also partly modified the O isotope composition of zircon. The low δ18O values of zircon in some samples may be partly acquired from the low δ18O hydrothermal fluid. The anomalously low δ18O values of -2.88‰ to -7.67‰ for quartz indicate that the hydrothermal fluid may be derived from the meteoric water of a cold paleoclimate. Therefore, the formation of low δ18O granite during the mid-Neoproterozoic at Xinkailing is correlated with rifting magmatism and the snowball earth event.
2005, 30(6): 673-684.
Abstract:
When the continental crust subducted to a certain depth (~90-110 km), almost no hydrous minerals existed in the plate, and nominally anhydrous minerals (NAMs, such as garnet, pyroxene, quartz) in UHP rocks became the main carriers transporting water into the mantle. Garnet and omphacite in UHP eclogites at Shuanghe, Dabieshan have been investigated by Micro-FTIR. The results demonstrate that all garnet and omphacite grains contain structural water occurring as hydroxyl (OH) with the content from 30×10-6 to 1860×10-6 (H 2O wt.) and from 360×10-6 to 620×10-6, respectively. The water content (300×10-6 to 750×10-6) of whole rocks suggests that UHP rocks can recycle at least several hundred ×10-6 water into the mantle by deep continental subduction. There are two types of water distribution characteristics within a single garnet grain, one is homogeneous and the other has higher water content in the core and lower in the rim. The lower water content in the rim possibly arises from hydrogen diffusion in response to a sharp decrease in pressure during the exhumation, and the expelled hydrogen may be the source of early-stage retrograde fluid. The ratio of water content in omphacite and garnet is between 0.5 and 3.5.
When the continental crust subducted to a certain depth (~90-110 km), almost no hydrous minerals existed in the plate, and nominally anhydrous minerals (NAMs, such as garnet, pyroxene, quartz) in UHP rocks became the main carriers transporting water into the mantle. Garnet and omphacite in UHP eclogites at Shuanghe, Dabieshan have been investigated by Micro-FTIR. The results demonstrate that all garnet and omphacite grains contain structural water occurring as hydroxyl (OH) with the content from 30×10-6 to 1860×10-6 (H 2O wt.) and from 360×10-6 to 620×10-6, respectively. The water content (300×10-6 to 750×10-6) of whole rocks suggests that UHP rocks can recycle at least several hundred ×10-6 water into the mantle by deep continental subduction. There are two types of water distribution characteristics within a single garnet grain, one is homogeneous and the other has higher water content in the core and lower in the rim. The lower water content in the rim possibly arises from hydrogen diffusion in response to a sharp decrease in pressure during the exhumation, and the expelled hydrogen may be the source of early-stage retrograde fluid. The ratio of water content in omphacite and garnet is between 0.5 and 3.5.
2005, 30(6): 685-691.
Abstract:
The Jiuling granitic batholith in Jiangxi Province crops out at the southern margin of the Yangtze block, and it is the largest Neoproterozoic granitoid intrusion in South China. The granitic complex has long been considered to consist of Jinningian granites, Hercynian granites and Yanshanian granites. In this paper, a SHRIMP U-Pb zircon geochronological study on this complex has been performed on the basis of detailed field research. The 206Pb/238U age of granite thought to be formed in the Neoproterozoic is (828±8) Ma, while the age of one sample from Ganfang granite thought to be formed in the Hercynian is (820±10) Ma, which suggests there is no Hercynian granite in this area. Combined with other new geochronological data, it seems that there is no proof of Caledonian-Hercynian magmatic activity within the Yangtze block, which may infer that there is no Caledonian-Hercynian intrusion in the interior of the Yangtze block. The age of a sample thought to be formed in the Yanshanian is (151.4±2.4) Ma, which confirms previous results. A few inherited zircons record Mesoproterozoic age information, which may represent the ages of the granite source rocks.
The Jiuling granitic batholith in Jiangxi Province crops out at the southern margin of the Yangtze block, and it is the largest Neoproterozoic granitoid intrusion in South China. The granitic complex has long been considered to consist of Jinningian granites, Hercynian granites and Yanshanian granites. In this paper, a SHRIMP U-Pb zircon geochronological study on this complex has been performed on the basis of detailed field research. The 206Pb/238U age of granite thought to be formed in the Neoproterozoic is (828±8) Ma, while the age of one sample from Ganfang granite thought to be formed in the Hercynian is (820±10) Ma, which suggests there is no Hercynian granite in this area. Combined with other new geochronological data, it seems that there is no proof of Caledonian-Hercynian magmatic activity within the Yangtze block, which may infer that there is no Caledonian-Hercynian intrusion in the interior of the Yangtze block. The age of a sample thought to be formed in the Yanshanian is (151.4±2.4) Ma, which confirms previous results. A few inherited zircons record Mesoproterozoic age information, which may represent the ages of the granite source rocks.
2005, 30(6): 692-706.
Abstract:
Extremely 18O-depleted zircons from granitic gneisses, with δ18O values as low as -7.8‰, were found in the Zaobuzhen area in the Weihai region, at the northeastern end of the Sulu orogen. SHRIMP zircon U-Pb ages and the oxygen isotope compositions were determined for the Low-δ18O zircons. CL images reveal that the Low-δ18O zircons are primarily of igneous origin, but some grains underwent metamorphic recrystallization. The igneous zircons from a granitic gneiss sample yield a concordant U-Pb age of (760±49) Ma and an upper intercepted age of (751±27) Ma, indicating a protolith of Middle Neoproterozoic age. Metamorphic zircons from the same sample yield a concordant U-Pb age of (232±4) Ma and a lower intercepted age of (241±33) Ma, pointing to Triassic UHP metamorphism. Most of the igneous zircons have unusually variable δ18O values of -7.76‰ to 5.40‰, indicating that the gneiss protolith was intruded as Low-δ18O magma that was generated by the partial melting of altered rocks, which suffered intensive water-rock interaction with a Low-δ18O fluid at high temperatures during the Neoproterozoic. The preservation of extreme 18O-depletion in the zircons suggests that there is no remarkable oxygen isotope exchange between the metagranite and the mantle during the processes of Triassic subduction and exhumation. The protolith nature, metamorphic timing and oxygen isotope compositions of the granitic gneisses in the Weihai region are similar to those of granitic gneisses in the Qinglongshan area in the southwestern part of the Sulu orogenic belt, indicating that the gneisses along the Sulu orogenic belt share the same nature of protolith origin, water-rock interaction and UHP metamorphism. The present study provides tight constraints not only on the origin of extremely 18O-depleted zircons, but also on the protolith nature of granitic gneisses in the Sulu orogenic belt.
Extremely 18O-depleted zircons from granitic gneisses, with δ18O values as low as -7.8‰, were found in the Zaobuzhen area in the Weihai region, at the northeastern end of the Sulu orogen. SHRIMP zircon U-Pb ages and the oxygen isotope compositions were determined for the Low-δ18O zircons. CL images reveal that the Low-δ18O zircons are primarily of igneous origin, but some grains underwent metamorphic recrystallization. The igneous zircons from a granitic gneiss sample yield a concordant U-Pb age of (760±49) Ma and an upper intercepted age of (751±27) Ma, indicating a protolith of Middle Neoproterozoic age. Metamorphic zircons from the same sample yield a concordant U-Pb age of (232±4) Ma and a lower intercepted age of (241±33) Ma, pointing to Triassic UHP metamorphism. Most of the igneous zircons have unusually variable δ18O values of -7.76‰ to 5.40‰, indicating that the gneiss protolith was intruded as Low-δ18O magma that was generated by the partial melting of altered rocks, which suffered intensive water-rock interaction with a Low-δ18O fluid at high temperatures during the Neoproterozoic. The preservation of extreme 18O-depletion in the zircons suggests that there is no remarkable oxygen isotope exchange between the metagranite and the mantle during the processes of Triassic subduction and exhumation. The protolith nature, metamorphic timing and oxygen isotope compositions of the granitic gneisses in the Weihai region are similar to those of granitic gneisses in the Qinglongshan area in the southwestern part of the Sulu orogenic belt, indicating that the gneisses along the Sulu orogenic belt share the same nature of protolith origin, water-rock interaction and UHP metamorphism. The present study provides tight constraints not only on the origin of extremely 18O-depleted zircons, but also on the protolith nature of granitic gneisses in the Sulu orogenic belt.
2005, 30(6): 707-720.
Abstract:
The Mesozoic strata in Luanping, North Hebei Province are the most complete in North Hebei-West Liaoning, and the Luanping area is the only area where the deposit action in the Late Mesozoic is successive. A lot of researchers think the boundary of the J3-K1 and the tectonic framework transition interface in Mesozoic Era in North Hebei-West Liaoning are located between the Tuchengzi and Zhangiiakou formations in the Luanping area. The Mesozoic strata in Luanping from the lower to the upper are the Tiaojishan Formation, Tuehengzi Formation, Zhangjiakou Formation, Dabeigou Formation, Dadianzi Formation and the Xiguayuan Formation. U-Pb zircon dating of LA-ICP-MS obtained the age frame of the Mesozoic volcanic strata in the Luanping area. The top age of the Tiaojishan Formation is (162.8±3.2)Ma, the main forming period of the Tuehengzi Formation is (142.6±1.3)Ma-(136.4±1.9)Ma, the forming period of the Zhangjiakou Formation is (135.7±1.8)Ma--(135.2±2.3)Ma, and the age of the volcanic rocks in the top of the Dadianzi Formation is (131.4±3. 7)Ma--(130.2±3. 0)Ma. Analysis shows that the rare earth element characteristics of zircons from the Tuehengzi Formation are very similar to those from the Zhangiiakou Formation, but are very different from those from the Tiaojishan Formarion. Combined with the field characteristics of the strata, the following conclusions are drawn: (1) the time interval Detween the Tuchengzi and Tiaojishan formations is longer, and the magmatic sources of the volcanic rocks are very different; the forming period between the Tuchengzi and the Zhangjiakou formations is continuous, and the magmatic sources of the volcanic rocks are very similar. The results imply that the Tuchengzi and Zhangjiakou formations were formed in the same geological background, and so are not the boundary of the J3-K1 and the tectonic framework transition interface between the Tuchengzi and Zhangjiakou formations in the Luanping area. (2) The age of the volcanic rocks in the top of the Dadianzi Formation corresponds to that of the bottom of the Yixian Formation in the Beipiao-Yixian area in West Liaoning Province, and corresponds to that of the top of the Zhangiiakou Formation in Lingyuan, West Liaoning Province.
The Mesozoic strata in Luanping, North Hebei Province are the most complete in North Hebei-West Liaoning, and the Luanping area is the only area where the deposit action in the Late Mesozoic is successive. A lot of researchers think the boundary of the J3-K1 and the tectonic framework transition interface in Mesozoic Era in North Hebei-West Liaoning are located between the Tuchengzi and Zhangiiakou formations in the Luanping area. The Mesozoic strata in Luanping from the lower to the upper are the Tiaojishan Formation, Tuehengzi Formation, Zhangjiakou Formation, Dabeigou Formation, Dadianzi Formation and the Xiguayuan Formation. U-Pb zircon dating of LA-ICP-MS obtained the age frame of the Mesozoic volcanic strata in the Luanping area. The top age of the Tiaojishan Formation is (162.8±3.2)Ma, the main forming period of the Tuehengzi Formation is (142.6±1.3)Ma-(136.4±1.9)Ma, the forming period of the Zhangjiakou Formation is (135.7±1.8)Ma--(135.2±2.3)Ma, and the age of the volcanic rocks in the top of the Dadianzi Formation is (131.4±3. 7)Ma--(130.2±3. 0)Ma. Analysis shows that the rare earth element characteristics of zircons from the Tuehengzi Formation are very similar to those from the Zhangiiakou Formation, but are very different from those from the Tiaojishan Formarion. Combined with the field characteristics of the strata, the following conclusions are drawn: (1) the time interval Detween the Tuchengzi and Tiaojishan formations is longer, and the magmatic sources of the volcanic rocks are very different; the forming period between the Tuchengzi and the Zhangjiakou formations is continuous, and the magmatic sources of the volcanic rocks are very similar. The results imply that the Tuchengzi and Zhangjiakou formations were formed in the same geological background, and so are not the boundary of the J3-K1 and the tectonic framework transition interface between the Tuchengzi and Zhangjiakou formations in the Luanping area. (2) The age of the volcanic rocks in the top of the Dadianzi Formation corresponds to that of the bottom of the Yixian Formation in the Beipiao-Yixian area in West Liaoning Province, and corresponds to that of the top of the Zhangiiakou Formation in Lingyuan, West Liaoning Province.
2005, 30(6): 721-728.
Abstract:
A large number of studies of geochronology and geochemistry indicate that Mesozoic intermediate-felsic rocks from the Dabie orogen have originated from recycling of the subducted Yangtze continental crust itself. A REE partition simulation suggests that the Tianzhushan intermediate rocks can be generated by partial melting, in coupling with fractional crystallization, of dioritic gneiss which is similar in chemical compositions to the mafic rocks in the lower crust in North Dabie; partial melting of intermediate grey gneiss which is similar in chemical compositions to the middle crust in North Dabie can generate the Tianzhushan felsic rocks. Quantitative calculation of trace element content for the residue after partial melting suggests that intermediate-mafic granulites outcropped in the Dabie orogen are not the residue of partial melting. The residue formed by partial melting of thickened crust due to anomalous heat supply by mantle superwelling at Early Cretaceous is possibly easy to detach from the crust because of its high density, leading to the remove of mountain root and large scale uplift of the orogen.
A large number of studies of geochronology and geochemistry indicate that Mesozoic intermediate-felsic rocks from the Dabie orogen have originated from recycling of the subducted Yangtze continental crust itself. A REE partition simulation suggests that the Tianzhushan intermediate rocks can be generated by partial melting, in coupling with fractional crystallization, of dioritic gneiss which is similar in chemical compositions to the mafic rocks in the lower crust in North Dabie; partial melting of intermediate grey gneiss which is similar in chemical compositions to the middle crust in North Dabie can generate the Tianzhushan felsic rocks. Quantitative calculation of trace element content for the residue after partial melting suggests that intermediate-mafic granulites outcropped in the Dabie orogen are not the residue of partial melting. The residue formed by partial melting of thickened crust due to anomalous heat supply by mantle superwelling at Early Cretaceous is possibly easy to detach from the crust because of its high density, leading to the remove of mountain root and large scale uplift of the orogen.
2005, 30(6): 729-737.
Abstract:
Pb isotope compositions play an important role in tectonic division and evolution, block interaction and identifying different block relations in the crust. Our study shows that Pb isotope compositions of the high-pressure (HP) metamorphic rocks in different parts of Tongbai-Dabie orogenic belt, consisting of two-mica albite gneisses and eclogites, are similar and characterized by upper crustal Pb isotope compositions. They have 206pb/204Pb= 17. 599 - 18. 310, 207pb/204Pb= 15. 318 - 15. 615, 208pb/204Pb=37. 968 - 39. 143. The Pb isotope compositions of the HP metamorphic rocks both in Tongbai and Dahie areas are consistent. It is suggested that the HP metamorphic rocks both in Tongbai and Dabie areas belong to the same lithotectonic unit. In general, the Pb isotope compositions of the HP metamorphic rocks are higher than those of the ultrahigh pressure (UHP) metamorphic rocks. It is evident that the Pb isotope compositions of the rocks from the subducted Yangtze continental crust in Tongbai-Dabie orogenic belt show a regular variation from lower crust to upper crust, from lower radiogenic abundances in the lower crust to higher abundances in the upper crust. The foliated garnet-bearing granites, in-truded into the HP metamorphic rocks in different parts of the orogenic belt have similar Pb isotope compositions. They have 206pb/204Pb= 17. 128 - 17. 434, 207pb/204Pb= 15. 313 - 15. 422 and 208pb/204Pb= 37. 631 - 38. 122, which are obviously different from the compositions of the HP metamorphic rocks but similar to those of the UHP metamorphic rocks and the foliated garnet-bearing granites in the UHP metamorphic rocks. This shows that the granites both in the HP and UHP metamorphic rocks have a common magma source. The foliated garnet-bearing granites have the geochemical characteristics of A-type granites, suggesting that the magma of these granites in the UHP and HP metamorphic rocks were derived from the partial melting of the retro-metamorphosed UHP metamorphic rocks exhumed into the middle to lower crust.
Pb isotope compositions play an important role in tectonic division and evolution, block interaction and identifying different block relations in the crust. Our study shows that Pb isotope compositions of the high-pressure (HP) metamorphic rocks in different parts of Tongbai-Dabie orogenic belt, consisting of two-mica albite gneisses and eclogites, are similar and characterized by upper crustal Pb isotope compositions. They have 206pb/204Pb= 17. 599 - 18. 310, 207pb/204Pb= 15. 318 - 15. 615, 208pb/204Pb=37. 968 - 39. 143. The Pb isotope compositions of the HP metamorphic rocks both in Tongbai and Dahie areas are consistent. It is suggested that the HP metamorphic rocks both in Tongbai and Dabie areas belong to the same lithotectonic unit. In general, the Pb isotope compositions of the HP metamorphic rocks are higher than those of the ultrahigh pressure (UHP) metamorphic rocks. It is evident that the Pb isotope compositions of the rocks from the subducted Yangtze continental crust in Tongbai-Dabie orogenic belt show a regular variation from lower crust to upper crust, from lower radiogenic abundances in the lower crust to higher abundances in the upper crust. The foliated garnet-bearing granites, in-truded into the HP metamorphic rocks in different parts of the orogenic belt have similar Pb isotope compositions. They have 206pb/204Pb= 17. 128 - 17. 434, 207pb/204Pb= 15. 313 - 15. 422 and 208pb/204Pb= 37. 631 - 38. 122, which are obviously different from the compositions of the HP metamorphic rocks but similar to those of the UHP metamorphic rocks and the foliated garnet-bearing granites in the UHP metamorphic rocks. This shows that the granites both in the HP and UHP metamorphic rocks have a common magma source. The foliated garnet-bearing granites have the geochemical characteristics of A-type granites, suggesting that the magma of these granites in the UHP and HP metamorphic rocks were derived from the partial melting of the retro-metamorphosed UHP metamorphic rocks exhumed into the middle to lower crust.
2005, 30(6): 738-746.
Abstract:
The Areletuobie metabasic rocks occur in the Lower Carboniferous Jiangbasitao Formation, which consists of sedimentary fine clastic sequences on the south side of the Ertix-Mayin'ebo deep-large fault that marks the boundary between the Altay and Junggar orogenic belts. The metabasic rocks display relatively high TiO2 (1.15%-2.20%) and MgO contents (7.03%-9.54%), and relatively low SiO2 (45.72%-48.31%) and K2O contents (0.13%-0.41%). They exhibit slight LREE enrichment on the chondrite-normalized REE patterns without Eu anomalies. Their MORB-normalized trace element patterns are characterized by large ion lithophile element (LILE) enrichment with significantly positive Th anomalies, slightly negative Nb anomalies and high Nb concentrations (2.6×10-6-8.7×10-6), whereas their high field strength element (HFSE) contents resemble those of MORB. The Zr/Nb ratios range from 18.9 to 32.7, similar to those of MORB. In contrast, their Ti/V ratios are higher than those of MORB. All of these features suggest that they were generated by relatively low degrees of melting of MORB-like depleted mantle source. Their εNd(t) values range from 7.40 to 8.35, lower than those of MORB, but higher than those rocks derived from oceanic island and intracontinental basalts. Consequently, the metabasic rocks have the somewhat transitional features of the volcanic rocks between a volcanic arc and an oceanic island. It is therefore inferred that they were formed from a back-arc basin, and represent the products in the early stage of back-arc basin spreading.
The Areletuobie metabasic rocks occur in the Lower Carboniferous Jiangbasitao Formation, which consists of sedimentary fine clastic sequences on the south side of the Ertix-Mayin'ebo deep-large fault that marks the boundary between the Altay and Junggar orogenic belts. The metabasic rocks display relatively high TiO2 (1.15%-2.20%) and MgO contents (7.03%-9.54%), and relatively low SiO2 (45.72%-48.31%) and K2O contents (0.13%-0.41%). They exhibit slight LREE enrichment on the chondrite-normalized REE patterns without Eu anomalies. Their MORB-normalized trace element patterns are characterized by large ion lithophile element (LILE) enrichment with significantly positive Th anomalies, slightly negative Nb anomalies and high Nb concentrations (2.6×10-6-8.7×10-6), whereas their high field strength element (HFSE) contents resemble those of MORB. The Zr/Nb ratios range from 18.9 to 32.7, similar to those of MORB. In contrast, their Ti/V ratios are higher than those of MORB. All of these features suggest that they were generated by relatively low degrees of melting of MORB-like depleted mantle source. Their εNd(t) values range from 7.40 to 8.35, lower than those of MORB, but higher than those rocks derived from oceanic island and intracontinental basalts. Consequently, the metabasic rocks have the somewhat transitional features of the volcanic rocks between a volcanic arc and an oceanic island. It is therefore inferred that they were formed from a back-arc basin, and represent the products in the early stage of back-arc basin spreading.
2005, 30(6): 747-760.
Abstract:
The Early Jurassic Yeba bimodal volcanic rocks occur among Lhasa, Dazi and Mozhugongka, with major rocks of metabasalt, basaltic ignimbrite, dacite, silicic tuff and volcanic breccia. SiO2 contents in lava rocks are 41%-50.4% and 64%-69%, belonging to calc-alkaline basalt and dacite. One notable feature of basalt is its low TiO2 contents, 0.66%- 1.01%, much lower than those of continental tholeiite. The ∑REE contents of basalt and dacite are 60.3-135 μg/g and 126.4-167.9 μg/g respectively. Both rocks have similar REE and other trace element characteristics, with enriched LREE and LILE relative to HREE and HFS, similar REE plots and without Eu anomaly. The basalts have depleted Ti, Ta, Zr and Nb and slightly negative Nb and Ta anomalies, with Nb*=0.54-1.17 and an average of 0.84. The dacites have depleted P and Ti and also slightly negative Nb and Ta anomalies, with Nb*=0.74-1.06 and an average of 0.86.The εNd(t), (87Sr/86Sr)i values for basalts are 0.96-10.03 and 0.704 3-0.706 4. These values for dacites are (-1.42) -1.08 and 0.703 8- 0.704 9 respectively. Trace elemental and isotopic studies suggest that both basalt and dacite originated from partial melting of the mantle wedge at different degrees above subducted belt. The spinel lherzolite in upper mantle is likely the source rocks for the bimodal volcanic rocks, which might have undergone selective metasomasis of crustal fluids. Metamorphism at late stage made influence on the LILE contents. The Yeba bimodal volcanic rocks formed in temporal extensional environment in mature island arc settings, resulting from the Indosinian Gangdise magmatic arc.
The Early Jurassic Yeba bimodal volcanic rocks occur among Lhasa, Dazi and Mozhugongka, with major rocks of metabasalt, basaltic ignimbrite, dacite, silicic tuff and volcanic breccia. SiO2 contents in lava rocks are 41%-50.4% and 64%-69%, belonging to calc-alkaline basalt and dacite. One notable feature of basalt is its low TiO2 contents, 0.66%- 1.01%, much lower than those of continental tholeiite. The ∑REE contents of basalt and dacite are 60.3-135 μg/g and 126.4-167.9 μg/g respectively. Both rocks have similar REE and other trace element characteristics, with enriched LREE and LILE relative to HREE and HFS, similar REE plots and without Eu anomaly. The basalts have depleted Ti, Ta, Zr and Nb and slightly negative Nb and Ta anomalies, with Nb*=0.54-1.17 and an average of 0.84. The dacites have depleted P and Ti and also slightly negative Nb and Ta anomalies, with Nb*=0.74-1.06 and an average of 0.86.The εNd(t), (87Sr/86Sr)i values for basalts are 0.96-10.03 and 0.704 3-0.706 4. These values for dacites are (-1.42) -1.08 and 0.703 8- 0.704 9 respectively. Trace elemental and isotopic studies suggest that both basalt and dacite originated from partial melting of the mantle wedge at different degrees above subducted belt. The spinel lherzolite in upper mantle is likely the source rocks for the bimodal volcanic rocks, which might have undergone selective metasomasis of crustal fluids. Metamorphism at late stage made influence on the LILE contents. The Yeba bimodal volcanic rocks formed in temporal extensional environment in mature island arc settings, resulting from the Indosinian Gangdise magmatic arc.
2005, 30(6): 761-770.
Abstract:
Based on K-Ar isotopic ages (287.5-243.6 Ma), diabase dyke swarms of the Changmaohezi from western Liaoning, Northeast China, formed in the Late Paleozoic. These diabases can be divided into low Ti-Fe diabase and high Ti-Fe diabase. The low Ti-Fe diabases are characterized by low TiO2 (< 2%), FeOt (12.39%-15.33%), V (227-335 μg/g), Sc (24-36 μg/g) and high SiO2 (45.61%-47.72%), Al2O3 (12.51%-16.71%), MgO (6.66%-9.31%), K2O (0.57%-2.39%), Cr (107-177 μg/g), Ni (96-235 μg/g), and low Ti/Y (327-496), Ti/Zr (69-114), which are similar to continental flood basalts. The high Ti-Fe diabases are characterized by high TiO2 (5%-6%), FeOt (22.13%-22.16%), V(850-859 μg/g), Sc (51-52 μg/g), and low SiO2 (42.88%-44.90%), Al2O3 (11.53%-11.57%), MgO (5.15%- 5.29%), K2O (0.48%-0.51%), Cr (< 2 μg/g), Ni (< 30 μg/g), and high Ti/Y (1 046-1 106), Ti/Zr (250-263), which are similar to the Skaergaard intrusion. All these rocks are enriched in Rb, Th, U, Pb, Ti, and light REE, and depleted in Ba, Sr, P, Nb and Ta. They have well-evolved Nd and Sr isotope compositions with εNd(t) =-6.43 to -4.12 and εSr(t) =42.94-64.19. The diabases were probably derived from the enriched lithospheric mantle and underwent fractional crystallization and crust assimilation. The high Ti-Fe diabases and the low Ti-Fe diabases are from different crystal conditions, the former in a closed system or a relatively oxidized state.
Based on K-Ar isotopic ages (287.5-243.6 Ma), diabase dyke swarms of the Changmaohezi from western Liaoning, Northeast China, formed in the Late Paleozoic. These diabases can be divided into low Ti-Fe diabase and high Ti-Fe diabase. The low Ti-Fe diabases are characterized by low TiO2 (< 2%), FeOt (12.39%-15.33%), V (227-335 μg/g), Sc (24-36 μg/g) and high SiO2 (45.61%-47.72%), Al2O3 (12.51%-16.71%), MgO (6.66%-9.31%), K2O (0.57%-2.39%), Cr (107-177 μg/g), Ni (96-235 μg/g), and low Ti/Y (327-496), Ti/Zr (69-114), which are similar to continental flood basalts. The high Ti-Fe diabases are characterized by high TiO2 (5%-6%), FeOt (22.13%-22.16%), V(850-859 μg/g), Sc (51-52 μg/g), and low SiO2 (42.88%-44.90%), Al2O3 (11.53%-11.57%), MgO (5.15%- 5.29%), K2O (0.48%-0.51%), Cr (< 2 μg/g), Ni (< 30 μg/g), and high Ti/Y (1 046-1 106), Ti/Zr (250-263), which are similar to the Skaergaard intrusion. All these rocks are enriched in Rb, Th, U, Pb, Ti, and light REE, and depleted in Ba, Sr, P, Nb and Ta. They have well-evolved Nd and Sr isotope compositions with εNd(t) =-6.43 to -4.12 and εSr(t) =42.94-64.19. The diabases were probably derived from the enriched lithospheric mantle and underwent fractional crystallization and crust assimilation. The high Ti-Fe diabases and the low Ti-Fe diabases are from different crystal conditions, the former in a closed system or a relatively oxidized state.
