High-temperature diffusion of major elements may obscure the records of early and peak metamorphic stages in granulites,while trace elements are more likely to preserve these records due to their lower diffusion rates...High-temperature diffusion of major elements may obscure the records of early and peak metamorphic stages in granulites,while trace elements are more likely to preserve these records due to their lower diffusion rates.Thus,using calibrated REE-based thermobarometers has proved essential for reconstructing these key stages,drawing considerable attention and application from scholars.However,the precision of these thermobarometers depends on including both major and trace elements from coexisting minerals to define the correlation coefficients(A,B,and D) among mineral pairs,indicating that the elemental composition of these pairs can affect the results.Our study examines the mafic-ultramafic granulites in the southern granulite terrain,India,employing integrated methods such as petrography,mineral chemistry,phase equilibrium modeling,and REE-based thermobarometers.We aim to determine their metamorphic conditions and evolutionary history and to identify potential challenges in using REE-based thermobarometers.The garnet,clinopyroxene,and orthopyroxene in the mafic-ultramafic granulite samples display homogeneous compositional profiles,with pronounced Fe-Mg diffusion zones at the interfaces between garnet and clinopyroxene.Conversely,the profiles of trace elements within garnet and clinopyroxene are better preserved.Investigations into Fe-Mg exchange and randomly selected mineral pairs significantly influence the accuracy of REE-based thermobarometers.Fe-Mg exchange can increase in coefficient A,while decreasing coefficient B for light rare earth elements(LREEs) and increasing it for heavy rare earth elements(HREEs),ultimately resulting in overestimations when calculating REE-based thermobarometers.For example,selecting major compositions with an Ex(=(X_(Mg)^(C)-X_(Mg)^(0))/X_(Mg)^(0)×100%;where X_(Mg)^(C) is the value after Fe-Mg exchange,X_(Mg)^(0) is the value before Fe-Mg exchange;X_(Mg)=Mg/(Fe^(2+)+Mg)) value of ~10 for calculation using the REE-in-Grt-Cpx thermobarometer will result in pressures and temperatures being ~10 kbar and 30-40℃ higher than the true values.Random pairing,such as selections based solely on the core or rim of minerals with changes in trace elements,can severely impact the distribution coefficient D,resulting in substantial discrepancies in thermobarometric calculations and potentially producing anomalous results.Thus,to minimize the impact of these factors,it is necessary to first analyze the profiles of major and trace elements in coexisting minerals before applying the REE-based thermobarometers to evaluate the P-T conditions of granulites.Based on this analysis,maj or element compositions less affected by Fe-Mg exchange(such as avoiding the selection of major compositions at the boundaries of minerals) and in relative equilibrium in trace element compositions among coexisting minerals(same growth periods) should be selected for pairing.Furthermore,integrating additional methods should also be considered when applying the REE-based thermobarometers,to prevent the misinterpretation of the P-T conditions obtained.展开更多
With mineral-melt thermobarometers,reconstruction of P-T-depth history of magmas can be established for vol-canic rocks.The pillow lava of Hantangang River Basalt is suitable for the study as it bears narrow compositi...With mineral-melt thermobarometers,reconstruction of P-T-depth history of magmas can be established for vol-canic rocks.The pillow lava of Hantangang River Basalt is suitable for the study as it bears narrow compositional range resulting from little or no fractional crystallization or crustal assimilation and shows evidence of rapid magma ascent.The established thermodynamic model covers the pathway from the magma source depth to the eruption.The model shows that the pillow lava originated at the depths of~85-100 km by fluid ascent from a stagnant slab.This range corresponds to the depth that encompasses the uppermost asthenosphere to the lowermost lithosphere corresponding to the upper garnet to the lower spinel sta-bility fields of the mantle.Subsequently,the melt rose to~66-71 km depth where a primary magma reservoir was generated possibly due to existence of a possible local discontinuity within the upper mantle.The magma uprose rapidly from~61 to~20 km or even to a shallower depth with crystallization of dendritic clinopyroxene and titano-magnetite,due to dehydration of magma.Magma ascent slowed down near the surface possibly due to the volcanic channel split into two or more toward the vents.The model can be applied to other volcanic areas composed of less evolved rocks.展开更多
The granulite xenoliths are first found in Yingfengling pyroclastic rocks of Leizhou region, Guangdong Province. Of them high_pressure garnet granulite xenolith found is very sparse in China. Garnet granulite is diffe...The granulite xenoliths are first found in Yingfengling pyroclastic rocks of Leizhou region, Guangdong Province. Of them high_pressure garnet granulite xenolith found is very sparse in China. Garnet granulite is different from pyroxene granulite in mineral assemblage and composition. \%P_T\% calculation shows that garnet granulite was formed at 1 130-1 160℃and 1.4-1.7 GPa, and pyriclasite at about 800℃and 0.65-0.80 GPa. High xenolith_derived paleogeotherm indicates Cenozoic rifting in Leizhou area. Granulites with varied mineral assemblages were formed at different depths by the metamorphism of the underplated basaltic melt.展开更多
基金supported by the National Natural Science Foundation of China (Grants Nos. 41890831 and 42302223)the State Key Laboratory of Continental Dynamics (Grant No. SKLCD-04)。
文摘High-temperature diffusion of major elements may obscure the records of early and peak metamorphic stages in granulites,while trace elements are more likely to preserve these records due to their lower diffusion rates.Thus,using calibrated REE-based thermobarometers has proved essential for reconstructing these key stages,drawing considerable attention and application from scholars.However,the precision of these thermobarometers depends on including both major and trace elements from coexisting minerals to define the correlation coefficients(A,B,and D) among mineral pairs,indicating that the elemental composition of these pairs can affect the results.Our study examines the mafic-ultramafic granulites in the southern granulite terrain,India,employing integrated methods such as petrography,mineral chemistry,phase equilibrium modeling,and REE-based thermobarometers.We aim to determine their metamorphic conditions and evolutionary history and to identify potential challenges in using REE-based thermobarometers.The garnet,clinopyroxene,and orthopyroxene in the mafic-ultramafic granulite samples display homogeneous compositional profiles,with pronounced Fe-Mg diffusion zones at the interfaces between garnet and clinopyroxene.Conversely,the profiles of trace elements within garnet and clinopyroxene are better preserved.Investigations into Fe-Mg exchange and randomly selected mineral pairs significantly influence the accuracy of REE-based thermobarometers.Fe-Mg exchange can increase in coefficient A,while decreasing coefficient B for light rare earth elements(LREEs) and increasing it for heavy rare earth elements(HREEs),ultimately resulting in overestimations when calculating REE-based thermobarometers.For example,selecting major compositions with an Ex(=(X_(Mg)^(C)-X_(Mg)^(0))/X_(Mg)^(0)×100%;where X_(Mg)^(C) is the value after Fe-Mg exchange,X_(Mg)^(0) is the value before Fe-Mg exchange;X_(Mg)=Mg/(Fe^(2+)+Mg)) value of ~10 for calculation using the REE-in-Grt-Cpx thermobarometer will result in pressures and temperatures being ~10 kbar and 30-40℃ higher than the true values.Random pairing,such as selections based solely on the core or rim of minerals with changes in trace elements,can severely impact the distribution coefficient D,resulting in substantial discrepancies in thermobarometric calculations and potentially producing anomalous results.Thus,to minimize the impact of these factors,it is necessary to first analyze the profiles of major and trace elements in coexisting minerals before applying the REE-based thermobarometers to evaluate the P-T conditions of granulites.Based on this analysis,maj or element compositions less affected by Fe-Mg exchange(such as avoiding the selection of major compositions at the boundaries of minerals) and in relative equilibrium in trace element compositions among coexisting minerals(same growth periods) should be selected for pairing.Furthermore,integrating additional methods should also be considered when applying the REE-based thermobarometers,to prevent the misinterpretation of the P-T conditions obtained.
基金supported by the 2021 Research Project for UNESCO Hantangang River Global Geopark supported by Gyeonggi Provincial Office(Grant No.20210606641-00)Basic Science Research Program through the National Research Foundation of Korea(NRF)funded by the Ministry of Education(No.2019R1A6A1A03033167).
文摘With mineral-melt thermobarometers,reconstruction of P-T-depth history of magmas can be established for vol-canic rocks.The pillow lava of Hantangang River Basalt is suitable for the study as it bears narrow compositional range resulting from little or no fractional crystallization or crustal assimilation and shows evidence of rapid magma ascent.The established thermodynamic model covers the pathway from the magma source depth to the eruption.The model shows that the pillow lava originated at the depths of~85-100 km by fluid ascent from a stagnant slab.This range corresponds to the depth that encompasses the uppermost asthenosphere to the lowermost lithosphere corresponding to the upper garnet to the lower spinel sta-bility fields of the mantle.Subsequently,the melt rose to~66-71 km depth where a primary magma reservoir was generated possibly due to existence of a possible local discontinuity within the upper mantle.The magma uprose rapidly from~61 to~20 km or even to a shallower depth with crystallization of dendritic clinopyroxene and titano-magnetite,due to dehydration of magma.Magma ascent slowed down near the surface possibly due to the volcanic channel split into two or more toward the vents.The model can be applied to other volcanic areas composed of less evolved rocks.
文摘The granulite xenoliths are first found in Yingfengling pyroclastic rocks of Leizhou region, Guangdong Province. Of them high_pressure garnet granulite xenolith found is very sparse in China. Garnet granulite is different from pyroxene granulite in mineral assemblage and composition. \%P_T\% calculation shows that garnet granulite was formed at 1 130-1 160℃and 1.4-1.7 GPa, and pyriclasite at about 800℃and 0.65-0.80 GPa. High xenolith_derived paleogeotherm indicates Cenozoic rifting in Leizhou area. Granulites with varied mineral assemblages were formed at different depths by the metamorphism of the underplated basaltic melt.
