Mineral Inclusions In Garnet Research Articles

広域変成作用論の革新的変貌

The discovery of ultrahigh-pressure rocks from collision-type orogenic belts has revolu-tionized the classic interpretation of (1) progressive and retrogressive metamorphism recorded on surface exposures of regional metamorphic belts, (2) geochronology of that metamorphism, (3) origin of metamorphic textures, (4) P-T-t path, (5) metamorphic facies series, (6) exhumation model, and (7) role of fluids during regional metamorphism. Based mainly on our recent studies of the Kokchetav, Dabie Shan, Indonesia, and the Franciscan and Sanbagawa belts, we point out or predict the following seven revolutionary paradigm shifts.So-called mineral isograds defined on the maps of regional metamorphic belts were a misunderstanding of the progressive dehydration reaction during subduction, because extensive late-stage hydration has mostly obliterated the progressive minerals in pelitic-psammitic, and metabasic rocks. Progressively zoned garnet has survived as the sole progressive mineral that was unstable with the majority of matrix-forming minerals. The classic Barrovian isograds should be carefully re-examined.The well-documented SHRIMP chronology of spot-dating zoned zircons with index minerals from low-P in the core, through HP-UHP in the mantle to low-P on the rim clearly shows that the slow exhumation speed of 23-40 m.y. from mantle depth to mid-crustal level was followed by mountain building with doming at latest stage. Extensive hydration of the UHP-HP unit occurred due to fluid infiltration underneath, when the UHP-HP unit intruded a low-grade to un-metamorphosed unit at a mid-crustal level.Most deformation textures such as mineral lineations, porphyroblasts, pull-apart or boudinaged amphiboles, formed during extensive hydration at the late-stage, hence do not indicate a progressive stress regime.The P-T-t path determined by thermobarometry using mineral inclusions in garnet and forward-modeling of garnet zoning, independent of the matrix minerals, indicates an anticlockwise path in the P-T space, and it follows an identical P-T change in the metamorphic facies series. This is consistent with the numerically calculated geotherm along the WadatiBenioff plane.Collision-type orogenic belts have long been regarded as being characterized by the intermediate-type metamorphic facies series. The kyanite-sillimanite-type is an apparent type facies series formed by late-stage extensive hydration. In contrast, the original high-P to ultrahigh-P type facies series with an anticlockwise kink-point at around 10 kb is a progressive type.A collision-type regional metamorphic belt crops out as a very thin unit sandwiched between overlying and underlying low-P or weakly metamorphosed units. The metamorphic belt has an aspect ratio (thickness vs width) of 1 : 100, and it extends for several hundreds to a thousand km. It resembles a thin mylonitic intrusion from the mantle extending a depth of from 100-200 km into the crustal rock unit. The underlying unit is thermally metamorphosed in the andalusite-sillimanite type facies series.The major reason for the misunderstanding of the progressive metamorphism in collisiontype orogenic belts is the underestimation of the role of fluids derived from the underlying the low-grade metamorphic unit, when juxtaposed at a mid-crustal level. The circulation of fluids at a plate boundary is more important than a P-T change.

Relict edenite in a garnet lherzolite from the Chinese Su-Lu UHP metamorphic terrane: Implications for metamorphic history

Study of mineral inclusions in garnet in orogenic peridotites provides important information on their metamorphic history. Similar study of mineral inclusions in pyroxene is also encouraging. Edenite is found as relict inclusions in phlogopite-clinopyroxene intergrowths forming the cores of clinopyroxene porphyroblasts in a garnet lherzolite at Zhimafang in the Chinese Su-Lu ultrahighpressure metamorphic terrane. Texture shows that the phlogopite and clinopyroxene were products of the breakdown of edenite. The composition of the edenite is represented by K-Ed 6.0 Ed 70.7 Fe- Ed 6.6 Ts 8.4 Fe-Ts 1.8 Tr 1.2 Cm 5.4 . Those of the clinopyroxene and phlogopite are, Ur 4.5 Jd 3.7 Ae 6.5 CaTs 2.2 Di 81.1 En 1.9 and Phl 86.0 Na-Phl 13.3 Na-Ann 0.7 , respectively. The edenite breakdown is accompanied by gains of Ca, Si, and Cr, and losses of H, Mg, Al, Fe, Na, and K. The hypothetical composition of clinopyroxene resulting from isochemical breakdown of edenite is higher in Jd, CaTs, and En but lower in Di components than the cores of clinopyroxene porphyroblasts. The calculated mode of phlogopite is much higher than measured. The gain of Si and loss of H can be explained by involvement of SiO 2 -enriched fluids during formation of the Cpx+Phl intergrowth, whereas the other gains and losses suggest the existence of an older generation clinopyroxene slightly more enriched in Ca and Cr and depleted in Na, Mg, and Al, which homogenized with the clinopyroxene produced by the breakdown of edenite. Interaction with fluids also increased the Ca/Mg ratio of the system. It is suggested that, after having been exhumed to a shallow level for the first time, the peridotite was infiltrated by crust-derived fluids resulting in the replacement of clinopyroxene by edenite. During subduction, the edenite was replaced by clinopyroxene + phlogopite. Relics of edenite in clinopyroxene were able to survive ultrahigh-pressure metamorphism.

P–T evolution of glaucophane–omphacite bearing HP–LT rocks in the western Tianshan Orogen, NW China:new evidence for ‘Alpine‐type’ tectonics

AbstractThe late Palaeozoic western Tianshan high‐pressure /low‐temperature belt extends for about 200 km along the south‐central Tianshan suture zone and is composed mainly of blueschist, eclogite and epidote amphibolite/greenschist facies rocks. P–T conditions of mafic garnet omphacite and garnet–omphacite blueschist, which are interlayered with eclogite, were investigated in order to establish an exhumation path for these high‐pressure rocks. Maximum pressure conditions are represented by the assemblage garnet–omphacite–paragonite–phengite–glaucophane–quartz–rutile. Estimated maximum pressures range between 18 and 21 kbar at temperatures between 490 and 570 °C. Decompression caused the destabilization of omphacite, garnet and glaucophane to albite, Ca‐amphibole and chlorite. The post‐eclogite facies metamorphic conditions between 9 and 14 kbar at 480–570 °C suggest an almost isothermal decompression from eclogite to epidote–amphibolite facies conditions. Prograde growth zoning and mineral inclusions in garnet as well as post‐eclogite facies conditions are evidence for a clockwise P–T path. Analysis of phase diagrams constrains the P–T path to more or less isothermal cooling which is well corroborated by the results of geothermobarometry and mineral textures. This implies that the high‐pressure rocks from the western Tianshan Orogen formed in a tectonic regime similar to ‘Alpine‐type’ tectonics. This contradicts previous models which favour ‘Franciscan‐type’ tectonics for the southern Tianshan high‐pressure rocks.

Eclogite Occurrences in the Southern Tianshan High-Pressure Belt, Xinjiang, Western China

Eclogite pods and layers of eclogites in mafic blueschists are the most prominent witnesses of eclogite facies metamorphism in the southern Tianshan high-pressure belt of Western China. The P-T evolution was deciphered in order to understand the geotectonic framework of these rocks. Geothermobarometric evaluation for matrix assemblages reveals peak metamorphic condition between 14 and 21 kbar at 500 to 600°C. Prograde growth zoning and mineral inclusions in garnet are evidence for a clockwise P-T path. The presence of intimate eclogite/metabasic blueschist interlayering and the high P-T estimates suggest that the eclogite-facies rocks from the southern Tianshan high-pressure belt formed in a tectonic regime similar to the Franciscan type. Therefore the southern Tianshan high-pressure belt is most important in order to resolve the Paleozoic tectonic evolution of central Asia.

Modification of garnet by fluid infiltration during regional metamorphism in garnet through sillimanite-zone rocks, Dutchess County, New York

Metapelitic rocks from a progressive metamorphic sequence in Dutchess County, New York, record evidence for reaction between garnet and fluids associated with quartz (::!:plagioclase) veins and for net transfer among garnet interiors, mineral inclusions, and fluid. Garnet-fluid reaction resulted in modification of preexisting garnet textures, compositions, and growth-zoning patterns. The textural and compositional record of garnet-fluid interaction varies with grade and with proximity of garnet to quartz veins. Garnet-zone rocks do not contain evidence for extensive reaction with vein-forming fluids. In upper stauroliteand kyanite-zone rocks, garnet that is crosscut by quartz veins contains a fluid-inclusion-filled region near the garnet-vein interface. This region truncates growth-zoning patterns and quartz inclusions. Fluid inclusions in garnet decrease in abundance away from quartz veins. In some rocks lacking quartz veins, fluid inclusions are concentrated in garnet interiors, and in particular near fractures, and are notably lacking around mineral inclusions in garnet. Inferences from textural and compositional features combined with thermobarometric results indicate that preexisting garnet grains were modified at T 525-550 °C and >4 kbar by one or more reactions involving fluids. The fluid pathways included foliationparallel channels in the rock as well as microcracks in mineral grains.

Quantitative determination of metamorphic reaction history: mass balance relations between groundmass and mineral inclusion assemblages in metamorphic rocks

Qualitative and quantitative information about metamorphic reaction history and PT paths may be obtained from mineral inclusions in garnet by comparing the mineralogy, distribution, and compositions of paragenetically-related inclusions with minerals in the groundmass assemblage. Using the algebraic technique of singular value decomposition (SVD), we document mass balance relations between inclusion and groundmass assemblages in metapelitic rocks from two metamorphic terranes that experienced different peak metamorphic conditions, and whose transition from inclusion to groundmass assemblage records different PT path segments relative to peak conditions. We calculate mass balances relating an inclusion assemblage consisting in part of armored relics of chloritoid to groundmass mineral assemblages in a kyanite-staurolite mica schist from the Solitude Range, British Columbia, and an inclusion assemblage of kyanite, staurolite, and rutile to groundmass minerals in a sillimanite-cordierite gneiss from the Skagit Gneiss, North Cascade Range, Washington. Mass balances for each rock are consistent with reaction histories inferred from petrographic observations. In the Solitude Range schist, the results of mass balance calculations are consistent with the growth of staurolite and garnet at the expense of chloritoid during prograde metamorphism and suggest that chlorite, although not preserved as an inclusion, was involved in initial staurolite growth. In the Skagit sillimanite gneiss, mass balance relations exist between the inclusion suite, which formed during high pressure metamorphism, and the associated groundmass assemblage, which equilibrated at high temperature but much lower pressure. Mass balance does not exist between the groundmass of the Skagit sillimanite gneiss and the groundmass of a nearby kyanite-staurolite schist that has been proposed as a possible lower-grade equivalent of the sillimanite-bearing rocks. These results indicate that, although compositional modification and selective preservation of minerals must be taken into account, mineral inclusion suites may nevertheless preserve enough compositional information to allow reconstruction of complete or nearly complete pre-existing assemblages. This information may not be retrievable from any other source if no lower-grade equivalents of the rocks of interest are exposed.

Prograde reaction histories deduced from compositional zonation and mineral inclusions in garnet from the Gassetts schist, Vermont

Prograde reaction histories deduced from compositional zonation and mineral inclusions in garnet from the Gassetts schist, Vermont