Accreted turbidite fans and remnant ocean basins in phanerozoic orogens: A template for a significant precambrian crustal growth and recycling process

Abstract

Convergent margin settings involving accretion of large turbidite fans with slivers of oceanic basement reflect important cites of continental crustal growth and recycling. Accreted crust consists of an upper layer of recycled arc and/or crustal detritus (turbidites) underlain by a layer of tectonically imbricated upper oceanic crust, and/or thinned continental crust, along with underplated magmatic material. When oceanic crust is converted to lower crust, it represents a juvenile addition to the continent. This twotiered accreted crust is commonly of average continental crustal thickness and isostatically balanced near sea level. The Paleozoic part of the Tasman Orogen (Lachlan-type) of eastern Australia is the archetypical example of a turbidite-dominated accretionary orogen. The Neoproterozoic Damaran Orogen of SW Africa is similar to the Lachlan-type except that it was incorporated into Gondwana via a continent-continent collision, whereas the Mesozoic Rangitatan Orogen of New Zealand illustrates the transition of convergent margin from a Lachlan-type to more typical accretionary wedge type orogen. The spatial and temporal variations in deformation, metamorphism, and magmatism across these orogens illustrate how large volumes of turbidite and their relict oceanic basement eventually become stable continental crust. The timing of deformation and metamorphism recorded in these rocks reflects the crustal thickening phase, whereas posttectonic granitoids and volcanic deposits constrain the timing of chemical maturation and cratonization. Cratonization and chemical maturation of continental crust is fostered in these orogenic settings because turbidites represent fertile sources for magma genesis, particularly for the S-type granites that are common in these orogens. The structural style and lithotectonic assemblages of the three Phanerozoic examples is remarkably similar to the Archean Jardine turbidites, which were accreted to the Wyoming craton by 2.8 Ga. Recognition of similar orogens in the Archean is important for the evaluation of crustal growth models, particularly for those based on detrital zircon age patterns, because crustal growth by accretion of the upper ocean crust or by underplating of mafic magmas does not readily result in the formation of zircon-bearing magmas at the time of accretion. This crust only produces significant zircon when and if it partially melts, which may be long after the actual time of accretion. Consequently, the significance of this process over earth history is distorted compared to more zircon-rich orogenic processes in probability densitybased analyses of crustal growth, but is recorded in Lu-Hf model ages of zircons from post-accretion magmas.