Decoding evolutionary history of provenance from beach placer monazites: A case study from Kanyakumari coast, southwest India

作者：Perumalsamy C; Bhadra Subhadip; Balakrishnan S^*

来源：Chemical Geology, 2016, 427: 83-97.

DOI：10.1016/j.chemgeo.2016.02.018

• Beach placer monazite
• EPMA chemical age
• T-DM model Nd age
• Southern Granulite Terrane
• Provenance proxy

Mineral chemical and geochronological studies of beach placer monazites from Kanyakumari coast, SW India were carried out to understand growth history of monazite and its bearing on tectono-thermal evolution of the source rock. In chemically zoned monazites lowest backscattered electron (BSE) response domains (dark gray) comprising the thick core region are mantled by highest BSE response rim domains (light gray region). Dark gray BSE domains are enriched in LREEs (La and Ce), U and Y, and depleted in Th and Pb compared to the light gray BSE domains. Chemical variability between these two domains can be linked with dominantly huttonitic substitution. Two U-Th-total Pb chemical age clusters between 555-578 Ma and 508-536 Ma were obtained respectively for low Th/U monazite cores and high Th/U monazite rims.
Sm-147/Nd-144 ratios in the analyzed monazites vary from 0.0733 to 0.1039 with an average value of 0.0902. High negative epsilon(Nd) (t = 0) values in the range of -26.5 to -30.8 indicate derivation of monazites from light LREE enriched igneous and metamorphic rocks. T-CHUR and T-DM model ages vary from 1687 Ma to 2364 Ma and 2035 Ma to 2702 Ma respectively. Average T-DM age of 2375 (+96/-44) Ma for placer monazites fits with similar to 2.4 Ga crustal accretionary episode in Southern Granulite Terrane (SGT). EPMA Sm-Nd ratios of beach placer monazites (similar to 0.153) are similar with the monazites in granites and granodiorites (0.150) of Nagercoil (NG) and Trivandrum Block (TB) of SGT. A comparison with available Th-U-total Pb EPMA monazite ages from various tectonic units within SGT suggests that growth history and crystallization age of monazites also correlate well with the Pan African granulite facies metamorphism (570 Ma) and post-peak evolution (535 Ma) of NG and TB.
The results obtained in this study augment the growing evidences that beach placer monazites can be used as a proxy for provenance study. A corollary of the study further confirms similar geological history of NG and TB since 2.1 Ga.

• 出版日期2016-6-1