Stratigraphic Relations, Kimberlite Emplacement And Lithospheric Thermal Evolution, Quirico Basin, Minas Gerais State, Brazil
Journal Article: Stratigraphic Relations, Kimberlite Emplacement And Lithospheric Thermal Evolution, Quirico Basin, Minas Gerais State, Brazil
AbstractThe Quirico Basin covers an area of 10,000 km(2) and is situated to the west of the conventionally defined southwestern margin of the Archean Sao Francisco craton in Minas Gerais State, Brazil. The sedimentary infill of the Quirico Basin consists of lightly metamorphosed shallow marine elastic bedrock sediments of the Bambui Group (similar to650 +/- 15 Ma), unconformably overlain by Early Cretaceous terrigenous lacustrine (Quirico Formation), alluvial fan (Abaete Formation) and fluvial/aeolian (Tres Barras Formation) deposits of the Areado Group. Rare kimberlites and ubiquitous kamafugites of the Alto Paranaiba Igneous Province (APIP) erupted through the recently deposited sediments of the Quirico Basin in the time period 95-61 Ma. The 120-m-thick Mata da Corda Group overlies the Late Cretaceous Areado Group over an area of 8000 km(2) and is composed largely of extrusive kamafugite and related volcanosedimentary material. Unusually large diamonds with proximal surface features and population characteristics are well known to occur in rivers and streams that drain the stratigraphic succession in the Quirico Basin, prompting the search for their presumably local primary source(s) and a possibly associated Archean basement or cratonic root. Conceptual exploration models for this setting may in part be based on the diamondiferous 120 Ma Canastra and 95 Ma Tres Ranchos kimberlites, but require reconciliation with the observed abundance of 85-61 Ma old diamond-free kamafugites. Field relations and carefully controlled stratigraphic samples show that a distinctive mantle-derived indicator mineral suite occurs in the Maxixe Member, a volcaniclastic breccia unit that occurs at the base of the Mata da Corda Group. A detailed thermobarometric comparison of mantle-derived xenocrystic clinopyroxene compositions from this member with clinopyroxene populations derived from kimberlites and kamafugites situated in the Quirico Basin shows a distinct and abrupt change in geothermal conditions at the onset of kamafugite magmatism. Kimberlites entrained fragments of garnet peridotite along a cool, craton-like geotherm which intersected the diamond stability field in the temperature range 880 - 1000 degreesC. Clinopyroxene xenocrysts in early-stage kamafugites (Maxixe Member) record heating by similar to240 degreesC, placing much of the deep lithosphere inside the graphite stability field. Clinopyroxenes contained in samples from common APIP kamafugites indicate the loss of the entire garnet-facies lithospheric section, estimated at 75 km thick at the time. This critical event fundamentally changed the lithospheric architecture and geothermal regime of the Quirico Basin from cratonic and "diamond-friendly" to noncratonic and "diamond-unfriendly" over a short time interval. The lithospheric evolution constrained here by judicious investigation of geological relationships during diamond exploration by Canabrava Diamond confirms the potential of the Quirico Basin to host diamondiferous kimberlites that have tapped cool cratonic lithosphere at times prior to eruption of Mata da Corda kamafugites. (C) 2004 Elsevier B.V. All rights reserved.
- George Read, Herman Grutter, Stewart Winter, Nigel Luckman, Frank Gaunt and Fernando Thomsen
- Published Journal
- Lithos, 2004
George Read,Herman Grutter,Stewart Winter,Nigel Luckman,Frank Gaunt,Fernando Thomsen. 2004. Stratigraphic Relations, Kimberlite Emplacement And Lithospheric Thermal Evolution, Quirico Basin, Minas Gerais State, Brazil. Lithos. (!) .