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Formation of ~2.5 Ga Sittampundi anorthosite complex in southern India: Implications to lower crustal stabilization of the Dharwar Craton.
- Source :
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Precambrian Research . Mar2021, Vol. 354, pN.PAG-N.PAG. 1p. - Publication Year :
- 2021
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Abstract
- • SIMS U-Pb zircon age of 2522 ± 12 Ma for anorthosite from the Sittampundi Complex. • Plagioclase 87Sr/86Sr ratios and chromite γ Os values argue for depleted mantle source. • Parental magma of anorthosite was contaminated by ancient mafic lower crust. • Anorthositic-gabbroic intrusions likely represent the lowermost crust of the Dharwar Craton. • Magmas underplating facilitated the Dharwar cratonization. Mantle-derived magmas at the base of the lower crust exerted a key control on late Archean cratonization in many continents. Since well-preserved, complete lower crustal section is rarely exposed, direct studies on the genetic link between mantle-derived magmas and cratonic lower crustal stabilization are inadequate. Cratonic lower crustal section is well-preserved in the southern margin of the Dharwar Craton (southern India), with a number of late Archean anorthositic-gabbroic complexes. Among these complexes, the Sittampundi anorthosite complex (SAC) consists of white- and dark-anorthosite (>60 vol%), gabbros, and ultramafic rocks. In this study, SIMS zircon U-Pb dating of the anorthosite revealed a minimum emplacement age of 2522 ± 12 Ma, similar to the chromite Os model ages (2528–2563 Ma) of the anorthosite-hosted chromitite. In-situ plagioclase (87Sr/86Sr) i ratios (0.70079–0.70100) of the dark anorthosite and the chromite γ Os (T) values (−0.2 to −0.4) of the chromitite suggest that the SAC was derived from a depleted mantle source. From the dark to white anorthosite, the (87Sr/86Sr) i ratios increase while the An contents decrease, suggesting crustal assimilation occurred during fractionation. Similarly, the mantle-like zircon δ18O values and relatively-wide ε Hf (T) (−2.1 to +8.4) range of the SAC anorthosite suggest that the parental magma had assimilated the ancient mafic lower crust. Emplacement ages of the SAC and published ages of the mafic/felsic granulites and charnockites altogether indicate that the anorthosites were formed during the Dharwar cratonization, and that the mantle-derived magma underplating may have led to extensive lower crustal melting. We argued that during underplating, high-density olivine-pyroxene cumulates (from fractionation of the mantle-derived magma) and partial-melting residues (in the overlying lower crust) mostly sank back to the underlying mantle. In contrast, the lower-density plagioclase and minor amphibole remained in the lower crust to form anorthositic-gabbroic sills. The magmas underplating and subsequent lower-crustal melting have likely made the cratonic lower crust more refractory and buoyant, which facilitated cratonization. [ABSTRACT FROM AUTHOR]
- Subjects :
- *ANORTHOSITE
*CHROMITE
*PLAGIOCLASE
*ULTRABASIC rocks
*URANIUM-lead dating
*MAGMAS
Subjects
Details
- Language :
- English
- ISSN :
- 03019268
- Volume :
- 354
- Database :
- Academic Search Index
- Journal :
- Precambrian Research
- Publication Type :
- Academic Journal
- Accession number :
- 148501848
- Full Text :
- https://doi.org/10.1016/j.precamres.2020.106012