16:45 〜 17:00
[SMP24-12] A geochemical perspective on tectonic setting and depositional environment of Precambrian metacarbonate rocks in collisional orogenic belts
キーワード：Metacarbonate rocks, East African Antarctic Orogen, Trace and rare earth element geochemistry, C, O, S, Sr, Nd and Pb isotope geochemistry
Carbonate rocks are the remnants of chemically precipitated sediments that preserve important records of paleo-oceanic continental margins, especially in Precambrian time. Although vulnerable to post-depositional alterations, pure carbonate rocks preserve geochemical proxies for understanding the depositional age and environment, and tectonic setting. A review of geochemical and isotopic characteristics is presented here to prove the versatility of metamorphosed carbonate rocks in deducing the premetamorphic signatures. The examples considered here are from East Antarctica, Sri Lanka, India, Madagascar and Africa that formed the part of the East African-Antarctic Orogen.
A comprehensive screening procedure involving field relations, C–O isotopic compositions, trace element distribution, and REE + Y patterns helped in selecting samples that preserve pre-metamorphic information. Shale-normalized REE patterns in most metacarbonate layers have typical signatures of open ocean deposition in a passive continental margin with variable fluvial input. In comparison to Phanerozoic equivalents, the absence of a Ce anomaly is most significant, whereas other REE variables such as (La/Sm)SN, (Sm/Yb)SN and (La/Yn)SN show positive and characteristic correlations with carbon and oxygen isotopes, Sri, epsilon Nd values and Nd model ages. Each depositional basin has distinctive geochemical parameters, especially those adjacent to preexisting continents and island arcs. From a compilation of the geochemical proxies of metacarbonate rocks from different terrains in the East Gondwana supercontinent, we can identify the paleogeographic distribution of carbonate rocks with respect to their apparent depositional age and their proximity to continents. The carbonate sedimentation progressed in a wide span across what is now the East African-Antarctic Orogenic Belt probably from the early Proterozoic to the Cryogenian. In summary, extracting trace and rare earth element characteristics and comparing them with carbon, oxygen, sulfur, strontium, neodymium and lead isotopes in carefully screened unaltered metacarbonate rocks in collisional orogenic belts can help in understanding the variations in the environment of deposition, age of deposition and the tectonic setting in which they were deposited, and in decoding the history of the lost oceans.
A comprehensive screening procedure involving field relations, C–O isotopic compositions, trace element distribution, and REE + Y patterns helped in selecting samples that preserve pre-metamorphic information. Shale-normalized REE patterns in most metacarbonate layers have typical signatures of open ocean deposition in a passive continental margin with variable fluvial input. In comparison to Phanerozoic equivalents, the absence of a Ce anomaly is most significant, whereas other REE variables such as (La/Sm)SN, (Sm/Yb)SN and (La/Yn)SN show positive and characteristic correlations with carbon and oxygen isotopes, Sri, epsilon Nd values and Nd model ages. Each depositional basin has distinctive geochemical parameters, especially those adjacent to preexisting continents and island arcs. From a compilation of the geochemical proxies of metacarbonate rocks from different terrains in the East Gondwana supercontinent, we can identify the paleogeographic distribution of carbonate rocks with respect to their apparent depositional age and their proximity to continents. The carbonate sedimentation progressed in a wide span across what is now the East African-Antarctic Orogenic Belt probably from the early Proterozoic to the Cryogenian. In summary, extracting trace and rare earth element characteristics and comparing them with carbon, oxygen, sulfur, strontium, neodymium and lead isotopes in carefully screened unaltered metacarbonate rocks in collisional orogenic belts can help in understanding the variations in the environment of deposition, age of deposition and the tectonic setting in which they were deposited, and in decoding the history of the lost oceans.