Accretion Of Juvenile Crust Research Articles

Age and geochemistry of Cambaí Complex, São Gabriel Terrane, Brazil: Arc-related TTG-like rocks

The Cambaí Complex TTG-like rock assemblage representing a Neoproterozoic juvenile magmatic arc is part of the São Gabriel Terrane that crops out in the SW domain of the Dom Feliciano Belt in S-Brazil. Geochemistry of dioritic, tonalitic, trondhjemitic, and granodioritic rocks of this complex indicate their low-medium-K calc-alkaline geochemical affinity being metaluminous and with fractional crystallization as the main magmatic process of formation. Whole-rock geochemistry enrichment in LILE as Sr and Ba in relations to HFSE as Y and Zr, with flat patterns in an REE spider diagram, with enrichment to chondrite ((La/Yb) N = 1.3 to 17.2) and low Eu anomaly (Eu/Eu* = 1.2), with 87Sr/86Sr(i) from 0.704260 to 0.706430, 143Nd/144Nd(t) from 0.512070 to 0.512080, Nd-TDM model age from 820 to 852 Ma, εNd(t) from 3.9 to 4.2 and 207Pb/204Pb and 206Pb/204Pb ranged from 15.5040 to 15.5776 and 17.5313 to 18.5696, suggest an immature magmatic arc-related rock setting. Geochronology U–Pb LA-ICP-MS in zircons from diorites shown Tonian crystallization ages (745 ± 4 Ma, 748 ± 5 Ma, 752 ± 4 Ma). Any model proposed to explain the history of the evolution of the West Gondwana Supercontinent needs to consider the accretion of juvenile crust from primitive mantle source with minor crustal contamination in this segment of the orogenic collage during the Neoproterozoic.

Accretion of juvenile crust at the Early Palaeozoic Antarctic margin of Gondwana: geochemical and geochronological evidence from granulite xenoliths

AbstractGeodynamic models for the Antarctic sector of the active Early Palaeozoic Palaeo‐Pacific margin of Gondwana are based on the nature and age of the deep crust of the Robertson Bay terrane, the outermost lithotectonic unit of the margin. As this crustal block is covered with thick turbidite deposits, the only way to probe the deep crust is through the analysis of granulite xenoliths from Cenozoic scoria cones. Low‐K felsic xenoliths yield the oldest (Middle Cambrian) laser‐probe U–Pb ages on zircon areas with igneous growth zoning. This finding, along with the positive whole‐rock εNd(500Ma), suggests that these felsic rocks derived from a juvenile magma formed during the Early Palaeozoic Ross orogenic cycle. Mafic xenoliths have geochemical‐isotopic compositions similar to those of modern primitive island arcs, suggesting the involvement of subducted oceanic crust in their magma genesis and accretion of juvenile crust at the Antarctic margin of Gondwana.

Anorthosites, Granulites and the Supercontinent Cycle

The pattern of occurrence of the massif-type anorthosites of the Proterozoic on a Rodinia reconstruction suggests that the geneses of the anorthosites, the associated granulites, and their chief repository- the Grenville age mobile belt - may be interrelated. This relationship has been examined within the broad geodynamic and spatial-temporal framework of the Proterozoic supercontinent. All over the Grenville age mobile belt, approximately within 1500 - 1000 Ma, two successive, back-to-back episodes can be recognized. A continent-continent collisional episode with massive sedimentation, great crustal shortening and thickening with large folds and nappes, metamorphism and calcalkaline magmatism, and accretion of juvenile crust was followed within little over a hundred million years or so by an extensional episode, beginning with large scale mantle-derived basic magma invasion, ponding and differentiation at the base of the thickened orogen, anorthosite diapirism and pervasive thermal overprinting of the lower and middle crust producing granulite belts. Cooling, unroofing and erosion of the orogen coincided with the later stages of the extension, which ended with splitting of the supercontinent. We have argued that the first episode marks the phase of closing in and amalgamation of the converging continental lithospheric plates and the second episode represents the phase of reversal, rifting, plate separation and drifting away of the post-split continental blocks. We believe, the supercontinental closing and opening cycles provide (a) realistic force fields and appropriate spatial-temporal and thermal-tectonic framework for the origin of the Proterozoic massif-type anorthosites and the associated granulites and (b) an explanation for the spatially overlapping but apparently conflicting evidence of both collisional and extensional tectonic signatures in the Grenville age orogenic belts.