A lot of liquid-gas and liquid-gas-solid inclusions were found in Pharkant jadeitites, northwestern Myanmar and their characteristics, geological setting and porphyroclastic jadeites with inclusions in them were descr...A lot of liquid-gas and liquid-gas-solid inclusions were found in Pharkant jadeitites, northwestern Myanmar and their characteristics, geological setting and porphyroclastic jadeites with inclusions in them were described in detail. The results analyzed by Raman spectrometer showed that the component of liquid-gas phase and solid phase (daughter minerals) in fluid inclusions is H2O + CH4 and jadeite separately. The results indicated that Pharkant jadeitites were crystallized from H2O + CH4 bearing jadeitic melt which may originate from mantle. The P-T conditions in which the jadeitites were crystallized were speculated to be T】650℃, P】1.5 GPa.展开更多
Jadeitites are formed either through direct precipitation from Na-Al-Si rich fluids(P-type),or by replacement of magmatic protoliths(R-type)in subduction zones.They are valuable targets for investigating the mobility ...Jadeitites are formed either through direct precipitation from Na-Al-Si rich fluids(P-type),or by replacement of magmatic protoliths(R-type)in subduction zones.They are valuable targets for investigating the mobility behavior and chemical composition of subduction zone fluids.The Rio San Juan Complex(RSJC)in the northern Dominican Republic hosts both P-and R-type jadeitites and jadeite-rich rocks,which provide ideal samples for addressing such issues.Here,we present trace element and Sr-Nd-O-Si isotope compositions of RSJC jadeitites and related rocks.Most samples show similar REE patterns,trace element distributions and δ^(18)O values to those of plagiogranite protoliths,indicating the predominance of R-type origin in RSJC.The P-type samples exhibit slightly higherδ^(30)Si values(−0.15‰to 0.25‰)than that of R-type samples(−0.20‰to 0.08‰),which place above the igneous array.The low(^(87)Sr/^(86)Sr)_(i)(0.70346 to 0.70505)and highεNd(t)values(4.6 to 6.8)of the P-type jadeitites and quartzites,along with relatively lowδ^(18)O values(4.7‰to 6.4‰)of their forming fluids,indicate that the fluids are likely derived from the altered basaltic crust rather than from oceanic sediment.However,the estimated jadeitite-and quartzite-forming fluids exhibit distinctδ^(30)Si values(0.76‰to 0.99‰and-0.48‰to-0.08‰,respectively),implying an evolution of the fluids that modified the Si isotopic compositions.Since fluid metasomatism and related desilication process could have lowered the whole-rock δ^(30)Si values,the heavy Si isotope compositions of the R-type samples are produced from the external fluids.Combing Rayleigh distillation and binary mixing simulations,we propose that fluids derived from altered oceanic crust obtained high δ^(30)Si values after crystallization of minerals enriched in light Si isotopes.The P-type jadeitites are formed through direct precipitation from this fluid.As the plagiogranite protoliths were continuously replaced by this fluid,the formed R-type samples(jadeitites and quartzites)also exhibit high δ^(30)Si values.Such rocks could significantly alter the Si isotope compositions of local mantle when they are deeply subducted at convergent plate margins.展开更多
This paper shows how a faulty approach to the study of mineral inclusions in zircon can lead to misleading interpretations of the geological context.We present and discuss two well-documented examples.Zircon grains se...This paper shows how a faulty approach to the study of mineral inclusions in zircon can lead to misleading interpretations of the geological context.We present and discuss two well-documented examples.Zircon grains separated from HP metamorphic jadeitite of the Rio San Juan Complex,Dominican Republic,and from UHP pyrope quartzite of the Dora Maira Massif,northern Italy,were studied using cathodoluminescence(CL)techniques,in combination with mineral inclusion and age data.In general,zircon from both localities shows inherited magmatic core domains with oscillatory zoning and metamorphic rims.The magmatic cores of zircon from the jadeitite yield ages of 115–117 Ma and host jadeite and omphacite which are of metamorphic origin and formed at about 78 Ma.Zircon from lawsonite blueschist,representing the country rock of the jadeitite,contains domains with oscillatory zoning that are nearly identical in age to the zircon cores from the adjacent jadeitite,and also contains younger metamorphic minerals such as lawsonite,albite,phengite(Si3.68),chlorite,and omphacite.Similar observations were made on the magmatic cores of zircon from the pyrope quartzite.These are about 275 Ma in age and host pyrope,phengite(Si3.55),talc,and kyanite,all of which formed during UHP metamorphism at about 35 Ma.Zircon from the biotite-phengite-gneiss country rock(metagranite)shows oscillatory zoning and yields ages that are identical to those of the magmatic cores of zircon from pyrope quartzite,which thus reflect granitic intrusion ages.The country-rock zircon also encloses metamorphic minerals with ages of about 35 Ma.Such minerals are,for example,garnet and phengite,as well as a polymineralic assemblage of clinopyroxene+garnet+phengite+quartz,that point to formation at UHP metamorphic conditions around 40 kbar/750℃.Based on these examples we suggest an effective approach centered on key evidence from CL studies to show that magmatic domains of zircon may actually contain pseudo-inclusions which were not entrapped during an early stage of formation,but were instead introduced during later metamorphic or metasomatic events along microcracks representing pathways for fluid influx.Cathodoluminescence microscopy is thus an excellent tool for avoiding such pitfalls by allowing distinction between true inclusions and pseudo-inclusions in zircon.展开更多
文摘A lot of liquid-gas and liquid-gas-solid inclusions were found in Pharkant jadeitites, northwestern Myanmar and their characteristics, geological setting and porphyroclastic jadeites with inclusions in them were described in detail. The results analyzed by Raman spectrometer showed that the component of liquid-gas phase and solid phase (daughter minerals) in fluid inclusions is H2O + CH4 and jadeite separately. The results indicated that Pharkant jadeitites were crystallized from H2O + CH4 bearing jadeitic melt which may originate from mantle. The P-T conditions in which the jadeitites were crystallized were speculated to be T】650℃, P】1.5 GPa.
基金supported by funds from the National Key Research and Development Program of China(Grant No.2024YFF0807302)National Natural Science Foundation of China(42273043,42173003)the International Visiting Professorship of USTC(2024BVR23).
文摘Jadeitites are formed either through direct precipitation from Na-Al-Si rich fluids(P-type),or by replacement of magmatic protoliths(R-type)in subduction zones.They are valuable targets for investigating the mobility behavior and chemical composition of subduction zone fluids.The Rio San Juan Complex(RSJC)in the northern Dominican Republic hosts both P-and R-type jadeitites and jadeite-rich rocks,which provide ideal samples for addressing such issues.Here,we present trace element and Sr-Nd-O-Si isotope compositions of RSJC jadeitites and related rocks.Most samples show similar REE patterns,trace element distributions and δ^(18)O values to those of plagiogranite protoliths,indicating the predominance of R-type origin in RSJC.The P-type samples exhibit slightly higherδ^(30)Si values(−0.15‰to 0.25‰)than that of R-type samples(−0.20‰to 0.08‰),which place above the igneous array.The low(^(87)Sr/^(86)Sr)_(i)(0.70346 to 0.70505)and highεNd(t)values(4.6 to 6.8)of the P-type jadeitites and quartzites,along with relatively lowδ^(18)O values(4.7‰to 6.4‰)of their forming fluids,indicate that the fluids are likely derived from the altered basaltic crust rather than from oceanic sediment.However,the estimated jadeitite-and quartzite-forming fluids exhibit distinctδ^(30)Si values(0.76‰to 0.99‰and-0.48‰to-0.08‰,respectively),implying an evolution of the fluids that modified the Si isotopic compositions.Since fluid metasomatism and related desilication process could have lowered the whole-rock δ^(30)Si values,the heavy Si isotope compositions of the R-type samples are produced from the external fluids.Combing Rayleigh distillation and binary mixing simulations,we propose that fluids derived from altered oceanic crust obtained high δ^(30)Si values after crystallization of minerals enriched in light Si isotopes.The P-type jadeitites are formed through direct precipitation from this fluid.As the plagiogranite protoliths were continuously replaced by this fluid,the formed R-type samples(jadeitites and quartzites)also exhibit high δ^(30)Si values.Such rocks could significantly alter the Si isotope compositions of local mantle when they are deeply subducted at convergent plate margins.
基金financially supported by the German Research Foundation (No. SCHE-517/10-1)
文摘This paper shows how a faulty approach to the study of mineral inclusions in zircon can lead to misleading interpretations of the geological context.We present and discuss two well-documented examples.Zircon grains separated from HP metamorphic jadeitite of the Rio San Juan Complex,Dominican Republic,and from UHP pyrope quartzite of the Dora Maira Massif,northern Italy,were studied using cathodoluminescence(CL)techniques,in combination with mineral inclusion and age data.In general,zircon from both localities shows inherited magmatic core domains with oscillatory zoning and metamorphic rims.The magmatic cores of zircon from the jadeitite yield ages of 115–117 Ma and host jadeite and omphacite which are of metamorphic origin and formed at about 78 Ma.Zircon from lawsonite blueschist,representing the country rock of the jadeitite,contains domains with oscillatory zoning that are nearly identical in age to the zircon cores from the adjacent jadeitite,and also contains younger metamorphic minerals such as lawsonite,albite,phengite(Si3.68),chlorite,and omphacite.Similar observations were made on the magmatic cores of zircon from the pyrope quartzite.These are about 275 Ma in age and host pyrope,phengite(Si3.55),talc,and kyanite,all of which formed during UHP metamorphism at about 35 Ma.Zircon from the biotite-phengite-gneiss country rock(metagranite)shows oscillatory zoning and yields ages that are identical to those of the magmatic cores of zircon from pyrope quartzite,which thus reflect granitic intrusion ages.The country-rock zircon also encloses metamorphic minerals with ages of about 35 Ma.Such minerals are,for example,garnet and phengite,as well as a polymineralic assemblage of clinopyroxene+garnet+phengite+quartz,that point to formation at UHP metamorphic conditions around 40 kbar/750℃.Based on these examples we suggest an effective approach centered on key evidence from CL studies to show that magmatic domains of zircon may actually contain pseudo-inclusions which were not entrapped during an early stage of formation,but were instead introduced during later metamorphic or metasomatic events along microcracks representing pathways for fluid influx.Cathodoluminescence microscopy is thus an excellent tool for avoiding such pitfalls by allowing distinction between true inclusions and pseudo-inclusions in zircon.
