[SMP37-05] C-O-Sr-Nd-Pb isotope geochemistry of metacarbonate rocks in continental collision zones as proxies for estimating the sedimentation age and understanding the tectonic setting of deposition
キーワード：Metacarbonate rocks, Carbon and oxygen isotopes, Lead isotopes
Carbonate rocks is a rare rock type that preserve extinct oceanic signatures. They helpd in the determination of depositional ages and tectonic environment of metasedimentary sequences in orogenic belts as well as in understanding the formation and evolution of supercontinents. Here we present a detailed study on carbon, oxygen, strontium, neodymium and lead isotope geochemistry of metacarbonate rocks from different East Gondwana continents. In particular, metacarbonate rocks from East Antarctica and Sri Lanka were studied in detail. The isotopic systems studied have distinct residence and mixing time in seawater, and are also controlled by the input from surrounding continents. We present here a model on the closure of the “Mozambique Ocean” prior to the final amalgamation of the Gondwana supercontinent purely based on the isotope geochemistry of metacarbonate rocks.
In order to obtain primary sedimentary geochemical information for the study area an extensive geochemical screening protocol for identifying post-depositional alterations, using oxygen isotopes, trace elements and REE + Y patterns was followed. In general, the metacarbonate rocks have positive oxygen isotopic composition, suggesting that these rocks were not affected by external infiltration of fluids during digenesis, dolomitization or metamorphism. Strontium isotope chemostratigraphy was applied for the selected samples. Depositional ages were estimated using established strontium isotope evolution curves. By using the εSr vs. εNd cross-plots it was possible to distinguish the typical seawater-rock mixing relationship in metacarbonate rocks associated with continents and oceanic crusts. Pb isotope analysis was carried out for the first time for the metacarbonate rocks in the EAAO belt. The results indicate that Pb isotopic composition showed significant difference within a single terrane. This might be due to the difference of initial composition, as well as the relationship to depositional basin and seawater composition.
Our results confirm the evidence of an oceanic island arc system and peripheral oceans to continents before the formation of the Gondwana supercontinent in the Neoproterozoic. By applying a multi-element isotope geochemical approach on chemostratigraphically well constrained metacarbonate rocks collected from continental collisional zones, and comparing the data with basement rocks from various neighbouring Gondwana continents, regional affinities could be established. Sr, Nd and Pb isotopic compositions provide important information about the depositional settings of sedimentary rocks and provide key information about the surrounding terrains of oceanic and continental affinities during the time of deposition. This can lead to a better understanding of paleo-tectonic settings of crustal fragments that assemble to form supercontinents.
[1] P.L. Dharmapriya, K. Sanjeeva, A. Galli, B-X. Su, N.D. Subasinghe, C.B. Dissanayake, Lithos, 200-223 (2015) 310-317.
[2] N. Otsuji, M. Satish-Kumar, A. Kamei, N. Tsuchiya, T. Kawakami, M. Ishikawa, G.H. Grantham, Precambrian Research, 234 (2013) 257-278.
[3] L.P. Knauth, L.M.J. Kennedy, Nature, 460 (2009) 728-732.
[4] G.P. Halverson, B.P. Wade, M.T. Hurtgen, K.M. Barovich, Precambrian Research, 182 (2010) 337-350.
[5] N. Otsuji, M. Satish-Kumar, A. Kamei, E. Takazawa, N. Tsuchiya, G.H. Grantham, T. Kawakami, M. Ishikawa, Y. Osanai, Journal of Mineralogical and Petrological Sciences 111 (2015) 170-180.
In order to obtain primary sedimentary geochemical information for the study area an extensive geochemical screening protocol for identifying post-depositional alterations, using oxygen isotopes, trace elements and REE + Y patterns was followed. In general, the metacarbonate rocks have positive oxygen isotopic composition, suggesting that these rocks were not affected by external infiltration of fluids during digenesis, dolomitization or metamorphism. Strontium isotope chemostratigraphy was applied for the selected samples. Depositional ages were estimated using established strontium isotope evolution curves. By using the εSr vs. εNd cross-plots it was possible to distinguish the typical seawater-rock mixing relationship in metacarbonate rocks associated with continents and oceanic crusts. Pb isotope analysis was carried out for the first time for the metacarbonate rocks in the EAAO belt. The results indicate that Pb isotopic composition showed significant difference within a single terrane. This might be due to the difference of initial composition, as well as the relationship to depositional basin and seawater composition.
Our results confirm the evidence of an oceanic island arc system and peripheral oceans to continents before the formation of the Gondwana supercontinent in the Neoproterozoic. By applying a multi-element isotope geochemical approach on chemostratigraphically well constrained metacarbonate rocks collected from continental collisional zones, and comparing the data with basement rocks from various neighbouring Gondwana continents, regional affinities could be established. Sr, Nd and Pb isotopic compositions provide important information about the depositional settings of sedimentary rocks and provide key information about the surrounding terrains of oceanic and continental affinities during the time of deposition. This can lead to a better understanding of paleo-tectonic settings of crustal fragments that assemble to form supercontinents.
[1] P.L. Dharmapriya, K. Sanjeeva, A. Galli, B-X. Su, N.D. Subasinghe, C.B. Dissanayake, Lithos, 200-223 (2015) 310-317.
[2] N. Otsuji, M. Satish-Kumar, A. Kamei, N. Tsuchiya, T. Kawakami, M. Ishikawa, G.H. Grantham, Precambrian Research, 234 (2013) 257-278.
[3] L.P. Knauth, L.M.J. Kennedy, Nature, 460 (2009) 728-732.
[4] G.P. Halverson, B.P. Wade, M.T. Hurtgen, K.M. Barovich, Precambrian Research, 182 (2010) 337-350.
[5] N. Otsuji, M. Satish-Kumar, A. Kamei, E. Takazawa, N. Tsuchiya, G.H. Grantham, T. Kawakami, M. Ishikawa, Y. Osanai, Journal of Mineralogical and Petrological Sciences 111 (2015) 170-180.