近20多年来,以南阿尔金为代表的多个典型超高压变质带中陆续发现了陆壳超深俯冲到或接近于斯石英稳定域地幔深度(>250~300km)并折返回地表的岩石学证据,代表了国际上大陆深俯冲与超高压作用研究领域的突破性新进展,并由此催生出了“...近20多年来,以南阿尔金为代表的多个典型超高压变质带中陆续发现了陆壳超深俯冲到或接近于斯石英稳定域地幔深度(>250~300km)并折返回地表的岩石学证据,代表了国际上大陆深俯冲与超高压作用研究领域的突破性新进展,并由此催生出了“陆壳超深俯冲作用”和“极超高压变质作用”的新概念。然而,由于以下两个方面的原因致使这些研究和认识还没有得到地学界的广泛认可。其一,这些岩石学证据主要是来自一些具有溶解度实验资料支撑的特殊矿物的显微出溶结构等,但由于矿物出溶结构成因的复杂性或多解性,尤其是矿物出溶结构的高温高压实验(或矿物溶解度实验的反向实验)研究鲜有报道,使得对矿物显微出溶结构成因的解释及其指示的地质意义存在较大争议,甚至怀疑其是否是“出溶结构”。其二,实验岩石学资料表明,大陆地壳物质俯冲到>250km的地幔深度时会发生一系列矿物相变,最终成为以高密度斯石英、K-锰钡矿、石榴子石等为主要组成矿物的岩石,并导致其密度高于围岩地幔岩而失去浮力,因此250km的地幔深度被前人定义为陆壳岩石“永不回返的深度”(Depth of no return)。那么,超深俯冲到斯石英稳定域地幔深度(>250km)的陆壳岩石是如何折返到地表的?这既是困惑国际地球科学界的一道难题,也是陆壳岩石超深俯冲到斯石英稳定域地幔深度后折返回地表的认识未被广泛接受的另一重要理由。本文重点针对这两个关键科学问题,并围绕极超高压变质作用及其构造地质意义等衍生科学问题,概述了近年来我们和其他研究团队取得的一些重要新进展,主要包括:(1)陆壳超深俯冲到斯石英或相当于斯石英稳定域的地幔深度(>250~300km)形成极超高压变质岩石,然后再折返回地表的地质现象在全球可能具有一定的普遍性,极超高压变质岩石类型具有多样性;(2)有溶解度实验和出溶实验数据支撑的矿物显微出溶结构与指示压力的特征矿物(如柯石英、金刚石、斯石英等)一样,可作为超-极超高压变质条件的识别标志,南阿尔金先期依据先存斯石英出溶蓝晶石+尖晶石等证据获得的关于陆壳岩石可俯冲到斯石英稳定域地幔深度并折返回地表的结论是可靠的;(3)俯冲陆壳与洋壳板片在>250km的地幔深度仍未断离,是控制陆壳超深俯冲与引发极超高压变质的必要条件之一;(4)加热使斯石英相变为柯石英导致岩石密度的减小,是超深俯冲到斯石英稳定域地幔深度(>250~300km)长英质陆壳岩石折返的主要驱动力,合理地解释了超深俯冲到斯石英稳定域地幔深度陆壳岩石的折返机制;(5)大陆板片超深俯冲过程中发生的4次矿物相变使其密度逐渐增大,尤其是柯石英相转变为斯石英(>250km的地幔深度)后俯冲板片的密度会显著大于围岩地幔,从而引发超深俯冲陆壳板片的后撤或回卷(rollback),进而导致俯冲带上盘出现伸展以及软流圈地幔的上涌,而上涌的软流圈地幔又可能为超深俯冲到斯石英稳定域地幔深度的大陆板片的加热提供了热源,致使其中斯石英转变为柯石英而获得自折返的浮力;(6)以大陆深-超深俯冲与折返过程及其地质响应为主线,以陆壳成因超-极超高压岩石的峰期变质、退变质和深熔-岩浆作用的演化序列为时间坐标,可用来约束大陆深-超深俯冲作用形成的碰撞造山带演化过程中洋盆关闭、大陆俯冲-碰撞、造山带伸展垮塌并抬升剥蚀等关键事件的时间节点。另外,本文还提出了关于陆壳超深俯冲与极超高压变质作用深入研究面临挑战的一些科学问题及其思考。展开更多
Mineralogical data are presented for the peridotite xenoliths from Miocene(~19 Ma)Qingyuan basalts in the eastern North China Craton(NCC),with the aim of constraining on property of the sub-continental lithospheric ma...Mineralogical data are presented for the peridotite xenoliths from Miocene(~19 Ma)Qingyuan basalts in the eastern North China Craton(NCC),with the aim of constraining on property of the sub-continental lithospheric mantle(SCLM)beneath the northern Tan-Lu fault zone(TLFZ)during the Cenozoic.The Qingyuan peridotites are dominated by spinel lherzolites with moderate-Mg^(#)olivines(89.4 to 91.2),suggesting that the regional SCLM is mainly transitional and fertile.Light rare earth element(LREE)-depleted,slightly depleted and enriched clinopyroxenes(Cpx)are identified in different peridotites.Chemical compositions of the LREE-enriched Cpx and the presence of phlogopite suggest that the Qingyuan SCLM has experienced silicate-related metasomatism.The synthesis of available mineral chemical data of the mantle xenoliths across the NCC confirms the SCLM beneath the NCC is highly heterogeneous in time and space.The Mesozoic–Cenozoic SCLM beneath the TLFZ and neighboring regions are more fertile and thinner than that beneath the region away from the fault zone.The fertile and refractory peridotite xenoliths experienced varying degrees of silicate and carbonatite metasomatism,respectively.The spatial-temporal lithospheric mantle heterogeneity in composition,age and thickness suggest that the trans-lithosphere fault zone played an important role in heterogeneous replacement of refractory cratonic lithospheric mantle.展开更多
Field-and petrographic investigations,together with microanalytical major-and traceelement studies,were carried out on clinopyroxene and amphibole from high-Mg diorite in the subduction-related Chelyabinsk granitic ma...Field-and petrographic investigations,together with microanalytical major-and traceelement studies,were carried out on clinopyroxene and amphibole from high-Mg diorite in the subduction-related Chelyabinsk granitic massif to understand its petrogenesis and source.The clinopyroxene composition(high Mg#,Cr-content,sum of REE and Ti/Eu ratio;depletion in HREE;negative Eu-anomaly)indicates that it formed from a reduced melt derived from a mantle source metasomatized by fluids/melts having crustal affinity.Melt compositions in equilibrium with clinopyroxene and amphibole were calculated using solid/liquid partition coefficients.The high Nb/Y and Zr/Y ratio values of a liquid simulated from clinopyroxene,which appears to have very similar characteristics to sanukitoid melts,indicate a low degree of melting of the mantle source.Melt simulated from amphibole is more evolved and more felsic(dacitic).It displays a geochemical“amphibole fractionation”signature,indicating the peritectic transformation of clinopyroxene to amphibole in the lower crust.Rock textures and major element mineral compositions suggest that further amphibole was precipitated directly from the melt in the middle crust.The results show that the Chelyabinsk highMg diorite was probably formed as a cumulate from sanukitoid-like melt during its ascent and cooling below dacitic liquidus inside the amphibole stability field.展开更多
文摘近20多年来,以南阿尔金为代表的多个典型超高压变质带中陆续发现了陆壳超深俯冲到或接近于斯石英稳定域地幔深度(>250~300km)并折返回地表的岩石学证据,代表了国际上大陆深俯冲与超高压作用研究领域的突破性新进展,并由此催生出了“陆壳超深俯冲作用”和“极超高压变质作用”的新概念。然而,由于以下两个方面的原因致使这些研究和认识还没有得到地学界的广泛认可。其一,这些岩石学证据主要是来自一些具有溶解度实验资料支撑的特殊矿物的显微出溶结构等,但由于矿物出溶结构成因的复杂性或多解性,尤其是矿物出溶结构的高温高压实验(或矿物溶解度实验的反向实验)研究鲜有报道,使得对矿物显微出溶结构成因的解释及其指示的地质意义存在较大争议,甚至怀疑其是否是“出溶结构”。其二,实验岩石学资料表明,大陆地壳物质俯冲到>250km的地幔深度时会发生一系列矿物相变,最终成为以高密度斯石英、K-锰钡矿、石榴子石等为主要组成矿物的岩石,并导致其密度高于围岩地幔岩而失去浮力,因此250km的地幔深度被前人定义为陆壳岩石“永不回返的深度”(Depth of no return)。那么,超深俯冲到斯石英稳定域地幔深度(>250km)的陆壳岩石是如何折返到地表的?这既是困惑国际地球科学界的一道难题,也是陆壳岩石超深俯冲到斯石英稳定域地幔深度后折返回地表的认识未被广泛接受的另一重要理由。本文重点针对这两个关键科学问题,并围绕极超高压变质作用及其构造地质意义等衍生科学问题,概述了近年来我们和其他研究团队取得的一些重要新进展,主要包括:(1)陆壳超深俯冲到斯石英或相当于斯石英稳定域的地幔深度(>250~300km)形成极超高压变质岩石,然后再折返回地表的地质现象在全球可能具有一定的普遍性,极超高压变质岩石类型具有多样性;(2)有溶解度实验和出溶实验数据支撑的矿物显微出溶结构与指示压力的特征矿物(如柯石英、金刚石、斯石英等)一样,可作为超-极超高压变质条件的识别标志,南阿尔金先期依据先存斯石英出溶蓝晶石+尖晶石等证据获得的关于陆壳岩石可俯冲到斯石英稳定域地幔深度并折返回地表的结论是可靠的;(3)俯冲陆壳与洋壳板片在>250km的地幔深度仍未断离,是控制陆壳超深俯冲与引发极超高压变质的必要条件之一;(4)加热使斯石英相变为柯石英导致岩石密度的减小,是超深俯冲到斯石英稳定域地幔深度(>250~300km)长英质陆壳岩石折返的主要驱动力,合理地解释了超深俯冲到斯石英稳定域地幔深度陆壳岩石的折返机制;(5)大陆板片超深俯冲过程中发生的4次矿物相变使其密度逐渐增大,尤其是柯石英相转变为斯石英(>250km的地幔深度)后俯冲板片的密度会显著大于围岩地幔,从而引发超深俯冲陆壳板片的后撤或回卷(rollback),进而导致俯冲带上盘出现伸展以及软流圈地幔的上涌,而上涌的软流圈地幔又可能为超深俯冲到斯石英稳定域地幔深度的大陆板片的加热提供了热源,致使其中斯石英转变为柯石英而获得自折返的浮力;(6)以大陆深-超深俯冲与折返过程及其地质响应为主线,以陆壳成因超-极超高压岩石的峰期变质、退变质和深熔-岩浆作用的演化序列为时间坐标,可用来约束大陆深-超深俯冲作用形成的碰撞造山带演化过程中洋盆关闭、大陆俯冲-碰撞、造山带伸展垮塌并抬升剥蚀等关键事件的时间节点。另外,本文还提出了关于陆壳超深俯冲与极超高压变质作用深入研究面临挑战的一些科学问题及其思考。
基金supported by funds from the Ministry of Science and Technology of the People's Republic of China(No.2019YFA0708603)NSFC(Nos.41973050,42288201,41930215)the Key Special Project for Introduced Talents Team of Southern Marine Science and Engineering Guangdong Laboratory(Guangzhou)(No.GML2019ZD0202)。
文摘Mineralogical data are presented for the peridotite xenoliths from Miocene(~19 Ma)Qingyuan basalts in the eastern North China Craton(NCC),with the aim of constraining on property of the sub-continental lithospheric mantle(SCLM)beneath the northern Tan-Lu fault zone(TLFZ)during the Cenozoic.The Qingyuan peridotites are dominated by spinel lherzolites with moderate-Mg^(#)olivines(89.4 to 91.2),suggesting that the regional SCLM is mainly transitional and fertile.Light rare earth element(LREE)-depleted,slightly depleted and enriched clinopyroxenes(Cpx)are identified in different peridotites.Chemical compositions of the LREE-enriched Cpx and the presence of phlogopite suggest that the Qingyuan SCLM has experienced silicate-related metasomatism.The synthesis of available mineral chemical data of the mantle xenoliths across the NCC confirms the SCLM beneath the NCC is highly heterogeneous in time and space.The Mesozoic–Cenozoic SCLM beneath the TLFZ and neighboring regions are more fertile and thinner than that beneath the region away from the fault zone.The fertile and refractory peridotite xenoliths experienced varying degrees of silicate and carbonatite metasomatism,respectively.The spatial-temporal lithospheric mantle heterogeneity in composition,age and thickness suggest that the trans-lithosphere fault zone played an important role in heterogeneous replacement of refractory cratonic lithospheric mantle.
文摘Field-and petrographic investigations,together with microanalytical major-and traceelement studies,were carried out on clinopyroxene and amphibole from high-Mg diorite in the subduction-related Chelyabinsk granitic massif to understand its petrogenesis and source.The clinopyroxene composition(high Mg#,Cr-content,sum of REE and Ti/Eu ratio;depletion in HREE;negative Eu-anomaly)indicates that it formed from a reduced melt derived from a mantle source metasomatized by fluids/melts having crustal affinity.Melt compositions in equilibrium with clinopyroxene and amphibole were calculated using solid/liquid partition coefficients.The high Nb/Y and Zr/Y ratio values of a liquid simulated from clinopyroxene,which appears to have very similar characteristics to sanukitoid melts,indicate a low degree of melting of the mantle source.Melt simulated from amphibole is more evolved and more felsic(dacitic).It displays a geochemical“amphibole fractionation”signature,indicating the peritectic transformation of clinopyroxene to amphibole in the lower crust.Rock textures and major element mineral compositions suggest that further amphibole was precipitated directly from the melt in the middle crust.The results show that the Chelyabinsk highMg diorite was probably formed as a cumulate from sanukitoid-like melt during its ascent and cooling below dacitic liquidus inside the amphibole stability field.
