Fluid evolution and exhumation path of the Trivandrum Granulite Block, southern India

Journal of Mineralogical and Petrological Sciences

Yoshikura

2004

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Ultrahigh temperature (UHT) granulites in Tonagh Island, Napier Complex, East Antarctica record peak metamorphic pressure temperature (P T) conditions of up to 9 kbar and 1100°C. Sapphirine, garnet, orthopyroxene and quartz in these rocks contain very high density fluid inclusions with melting temperatures close to that of pure CO 2 (−56.6°C) and homogenization temperatures down to −34.9°C translating into high CO 2 densities (up to 1.07 g/cm 3 ). The UHT granulites of Vizianagram in Eastern Ghats Belt, India which were subjected to extreme crustal metamorphism at >1000°C and 8 9 kbar also carry very high density (up to 1.15 g/cm 3 ) pure CO 2 inclusions in quartz adjacent to spinel rimmed by various coronas of sillimanite, orthopyroxene and garnet. Garnets in a granulite facies rock from Salem in southern India that equilibrated at peak P T conditions of 740 800°C and 9 11 kbar carry the highest density (1.17 g/cm 3 ) pure CO 2 yet reported from continental crust. This rock also contains very high density CO 2 inclusions in plagioclase and quartz. High density (0.998 g/cm 3 ) pure CO 2 rich fluid inclusions also occur abundantly within garnets from a mafic granulite in Ampitiya, central Highland Complex, Sri Lanka. The peak P T conditions of metamorphism of the mafic granulite are around 10.6 kbar and 985°C with subsequent rapid isothermal decompression along a clock wise path down to 5.5 kbar. In most of the cases above, the representative isochores for the CO 2 inclusions either penetrate through or pass very close to the P T windows defined from mineral phase equilibria indicating that the fluid inclusions were trapped at the time of peak or near post peak metamorphism. We thus recognize a group of "ultrahigh density" (UHD) CO 2 rich fluids that characterize UHT rocks and other granulites formed under extreme crustal metamorphism. These fluids contrast sharply with the lower density (generally <1.0 g/cm 3 ) CO 2 inclusions commonly reported from normal granulite facies rocks in various terrains. The presence of "synmetamorphic" UHD CO 2 within various minerals is consistent with the low water activities predicted by the mineral assemblages in these rocks. Although the source of the CO 2 is equivocal, mantle derived mafic magmas could have provided the heat and volatiles required for crustal metamorphism at extreme conditions displayed by these rocks.

Section: Discussionsupporting

confidence: 62%

Section: Timing Of Fluid Entrapmentmentioning

confidence: 99%

"Ultrahigh density" carbonic fluids in ultrahigh-temperature crustal metamorphism

Santosh

Journal of Mineralogical and Petrological Sciences

Yoshikura

2004

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“…Cenki et al (2002) proposed a clock wise P T path beginning at 6 7 kbar and 900 950°C. More recently, Fonarev et al (2003), based on combined mineral phase equilibria and fluid inclusion studies proposed a multiple P T path for TGB involving isothermal decompression starting at ca. 5 kbar and 800°C, followed by isobaric cooling and further re heating after exhumation down to pressures of 2 kbar.…”

Section: Discussionmentioning

confidence: 99%

“…The TGB has been the focus of various geological, petrological, isotopic and geochronological investigations for nearly two decades (e.g., Srikantappa et al, 1985;Chacko et al, 1987;Santosh, 1987, Santosh et al, 1990Jackson and Santosh, 1992;Raith and Srikantappa, 1993;Harley and Santosh, 1995;Braun et al, 1996;Nandakumar and Harley, 2000;Cenki et al, 2002;Fonarev et al, 2003). Among these, some studies have also proposed the possible high temperature history of the terrane (e.g., Braun et al, 1996;Chacko et al, 1996).…”

Section: Introductionmentioning

confidence: 99%

Spinel+Quartz association from the Kerala khondalites, southern India: evidence for ultrahigh-temperature metamorphism

Morimoto

Santosh

Journal of Mineralogical and Petrological Sciences

et al. 2004

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Granulite facies metapelites (khondalites) from Chittikara are characterized by the assemblage Grt + Sil + Bt + Crd + Spl + Kfs + Pl + Qtz ± graphite. The rocks show a series of mineral reactions such as: (1) Bt + Sil + Qtz = Grt + Crd ± Kfs, (2) Grt + Sil + Qtz = Crd, (3) Grt + Crd + Sil = Spl + Qtz , (4) Spl + Qtz = Crd, and (5) Grt + Sil = Spl + Crd. Spinel and quartz occur in direct grain contact and their stability considerations in appropriate reaction grids indicate > 950°C at 8 kbar. This, together with the ca. 1000°C temperatures derived from mesoperthites suggests ultrahigh temperature metamorphism in the khondalite belt. Electron microprobe data on the various minerals have been used to compute pressure temperature estimates from mineral phase equilibria thermobaromtery. Temperature estimates from spinel cordierite thermometry are around 900°C, while garnet cordierite thermometer yields an upper estimate of 818°C. Estimates based on garnet sillimanite plagioclase quartz barometer indicate pressures up to 8 kbar. Evaluation of the microstructures and reactions in the khondalites using available reaction grids for pelitic rocks provide evidence for peak metamorphism at high or ultrahigh temperatures followed by steep decompression from around 8 kbar down to 4 kbar. The P T path attests to a rapid exhumation history probably associated with the extensional collapse following collision of continental blocks during the assembly of Gondwana in the late Pan African.

“…The isochores shown in Figure 7 were obtained using fluid data calculated from the thermodynamic models of Bodnar and Vityk (1994) for aqueous fluids and Brown and Lamb (1986) for carbonic fluids. Such isochore calculations and comparison with P T path for characterization of peak metamorphic fluid have been applied to numerous granulite facies terranes (e.g., Tsunogae et al, 2002;Fonarev et al, 2003;Santosh and Tsunogae, 2003). In Figure 7, we plotted the highest density inclusions in garnet and quartz separately from other low density inclusions, as such high density inclusions may preserve primary fluids and are useful for discussion on the nature of synmetamorphic fluids.…”

Section: Microthermometrymentioning

confidence: 99%

Fluid inclusions study of Abukuma metamorphic rocks from the Hanazono district, northeast Japan

Ohyama

Journal of Mineralogical and Petrological Sciences

2007

A systematic fluid inclusion study on pelitic and granitic gneisses of the Cretaceous Abukuma metamorphic rocks, exposed in the Hanazono district, northeast Japan, identified three categories of fluid inclusions: primary aqueous inclusions in porphyroblastic garnet, primary to pseudosecondary carbonic inclusions in quartz, and secondary aqueous inclusions in quartz. The primary aqueous inclusions in garnet exhibit melting and homogenization temperatures of −2.5 to −0.3 °C and +168.2 to +360.0 °C, respectively. The dominant primary to pseudosecondary fluid inclusions in undeformed quartz display melting temperatures of −64.2 to −56.6 °C, indicating a CO 2 rich composition with additional CH 4 and/or N 2 . The carbonic inclusions can be divided into high density (1.121 1.153 g/cm 3 ) and medium density (0.683 1.111 g/cm 3 ) types. The fluid densities of the latter inclusions, when translated into isochores, indicate entrapment of CO 2 at the peak P T conditions experienced by rocks in the study area (2.8 5.2 kbar at 770 850 °C). In contrast, the isochores of high density category in quartz and aqueous inclusions in garnet do not intersect the expected P T trajectory, yet are consistent with the prograde high pressure condition of the Abukuma metamorphic rocks in the Takanuki district (8 10 kbar at 700 750 °C). Isochores calculated for carbonic inclusions in granitic gneiss intersect the P T path at 2 3 kbar at 650 °C, suggesting that the fluids were captured during the retrograde stage. It is inferred that H 2 O was the dominant fluid species during the prograde stage, while CO 2 was more abundant during the peak to retrograde stages of the Abukuma metamorphic belt. The prograde H 2 O bearing fluid was probably derived from the break down of hydrous minerals. Although the origin of carbonic fluid is not known, the presence of CO 2 rich fluid during peak metamorphism probably lowers the H 2 O activity in the rocks. The isochores computed for secondary aqueous inclusions in quartz give very low P T conditions (300 °C and 1.7 3.5 kbar), suggesting that the fluids are of retrograde origin.