Multiple Tectonothermal Events in the Granulite Blocks of Southern India Revealed from EPMA Dating: Implications on the History of Supercontinents

Journal of Mineralogical and Petrological Sciences

Tsunogae

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.

“…550 Ma (Harris et al, 1994;Bartlett et al, 1998;Santosh et al, 2003). EPMA dating of monazite from CHT has also yielded ages around 550 Ma (Bindu et al, 1998).…”

Section: Geology Of Chittikara Areamentioning

confidence: 99%

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

Morimoto

Journal of Mineralogical and Petrological Sciences

Tsunogae

et al. 2004

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“…Available geochronological data indicate multiple tectonothermal events in the granulite blocks and a protracted crustal evolution history, culminating in a major late Pan African (ca. 550 Ma) granulite facies event associated with the collision of continental assemblies and the birth of Gondwana supercontinent (Harris et al, 1994;Jayananda et al, 1995;Bartlett et al, 1998;Santosh et al, 2003).…”

Section: Geological Backgroundmentioning

confidence: 99%

Multistage orthopyroxene formation in ultrahigh-temperature granulites of Ganguvarpatti, southern India: implications for complex metamorphic evolution during Gondwana assembly

Tamashiro

Journal of Mineralogical and Petrological Sciences

Sajeev

et al. 2004

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Orthopyroxene sillimanite quartz granulites of Ganguvarpatti (GVP) from the northern Madurai Block in southern India record ultrahigh temperature (UHT) metamorphism and complex exhumation history. Orthopyroxenes in different textural setting in the GVP rocks shows marked variation in Al 2 O 3 and X Mg contents. Inclusions, cores of porphyroblasts and some of the discrete grains in the matrix possess the highest Al 2 O 3 contents (8.5 9.4 wt%). Orthopyroxenes occurring as fine to medium grained symplectitic intergrowths with cordierite show Al 2 O 3 values ranging from 7.6 8.2 wt%. Those which form symplectites with sapphirine have intermediate Al 2 O 3 values (6.9 7.5 wt%) while orthopyroxenes associated with spinel possess values between 7.3 7.5 wt%. Late orthopyroxene rim on biotite shows the lowest Al 2 O 3 range between 5.9 6.5 wt%. Orthopyroxenes from various textural associations define distinct P T slots in the Al in orthopyroxene thermometry based on the X Mg -X Al isopleths. Thus, orthopyroxene porphyroblasts and inclusions yield temperatures around 1050°C and pressure above 10 kbar. Orthopyroxene composition in orthopyroxene cordierite symplectite yields temperatures of 950 990°C at pressure above 8 kbar and those associated with sapphirine and spinel symplectites show 850 900°C at a pressure around 8 kbar. The late orthopyroxene rim around biotite yields temperatures around 850°C at 6 kbar. The UHT conditions attained by these rocks are also corroborated by garnet orthopyroxene thermometry, which provides an upper estimate of 1050 ± 50°C. The GVP rocks preserve a series of mineral reactions indicating ultrahigh temperature decompression. The reaction textures when evaluated in KFMASH petrogenetic grid indicate two stages of isobaric cooling and decompression, suggesting a complex exhumation history. The textures and mineral reactions observed in this study in conjunction with the available age data suggest that the multistage exhumation of the UHT rocks in Ganguvarpatti could be related to the orogenic events involving continent collision, crustal thickening and extensional collapse accompanying the step wise suturing of Gondwana supercontinent during the late Neoproterozoic.

“…Their results were interpreted to indicate the Neoarchean age of ~ 2530 Ma for the magmatic protoliths which was subjected to high -grade metamorphism at ~ 2480 Ma soon after the formation of the protoliths. On the other hand, the crustal fragments of the SGT to the south of the Salem Block preserve the common imprint of a major thermal event during the latest Neoproterozoic -Cambrian (e.g., Miller et al, 1996;Bartlett et al, 1998;Bhaskar Rao et al, 2003;Santosh et al, 2003Santosh et al, , 2006Collins et al, 2007aCollins et al, , 2007b.…”

Section: Introductionmentioning

confidence: 99%

Laser ablation ICP mass spectrometry for zircon U-Pb geochronology of metamorphosed granite from the Salem Block: Implication for Neoarchean crustal evolution in southern India

Sato

Journal of Mineralogical and Petrological Sciences

Tsunogae

et al. 2011

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We report here petrological characteristics of metamorphosed granite and metagabbro from the Kanja Malai hills, the southern margin of the Salem Block in southern India. Field observations suggest that both these rock types experienced high -grade metamorphism, and exhibit strong foliation. Many of cores of zircons extracted from the granite show an internal structure suggesting magmatic crystallization. In contrast, rims of the zircons commonly display metamorphic overgrowth. We also present geochronological data utilizing laser ablation ICP mass spectrometry technique on the zircons that show the weighted means of 207 Pb/ 206 Pb age of 2647 ± 11 Ma (error: 1σ) for the timing of magmatism, and 2442 ± 20 Ma (error: 1σ) for the subsequent high -grade metamorphism. Together with a previous geochronological study on charnockites from the Salem Block, our results also provide an important constraint on the Neoarchean magmatism (2.65 -2.53 Ga) in southern India, correlating the magmatism with a major global crust formation event during the Neoarchean.