High-temperature, low/medium-pressure clockwise P–T paths and melting in the development of regional migmatites: the role of crustal thickening and repeated plutonism

✍ Scribed by S. Jung

• Book ID: 102222375
• Publisher: John Wiley and Sons
• Year: 2000
• Tongue: English
• Weight: 294 KB
• Volume: 35
• Category: Article
• ISSN: 0072-1050
• DOI: 10.1002/gj.866

✦ Synopsis

Abstract

High‐temperature, low/medium‐pressure (c. 700 ± 50°C, c. 5 ± 1 kbar) migmatite terranes with abundant intrusions of granite from the Proterozoic to Palaeozoic Damara Orogen (Namibia) are mostly characterized by final near‐isobaric cooling paths in which substantial decompression occurred at high temperatures prior to cooling. The thermal peak, as inferred from Grt–Bt–Pl thermobarometry obtained from in situ migmatites, was reached between c. 500 and c. 520 Ma based on U‐Pb monazite ages and Sm‐Nd garnet–whole‐rock ages. The thermal peak was reached close to or shortly after the main deformation and was followed by intrusion of A‐type and S‐type granite magmas between c. 492–488 Ma and c. 492–484 Ma, respectively. On the other hand, high‐grade metapelitic and nebulitic country rocks record U‐Pb monazite ages of c. 538 Ma, c. 520–510 Ma and c. 487–470 Ma. Available age data indicate that the metamorphic evolution of the complex was not rapid but probably polyphase. Petrological studies suggest that the area was already at high metamorphic grade (Kfs + Sil‐present, muscovite‐absent, abundant residual migmatites) before the earliest granite intrusion which crystallized between 529 and 516 Ma. These results indicate that the features of granite intrusion are superimposed upon the regional metamorphic history. This conclusion is compatible with the observation that large contact aureoles are absent. A comprehensive geochronological study from the Oetmoed Migmatite Granite Complex, situated within the high‐grade central part of the orogen, shows that low/medium‐P, high‐T metamorphism was maintained by focusing mid‐crustal heat from repeated intrusion of crustally derived granite. High‐T regional metamorphism and intrusion of water‐undersaturated high‐temperature felsic melts initiated melting reactions which ultimately led to the co‐existence of in situ and intrusion‐related migmatites. It can be inferred that in situ migmatization was syntectonic (relative to crustal thickening), whereas formation of most of the mobile granitic magmas was post‐tectonic (relative to crustal thickening). Copyright © 2000 John Wiley & Sons, Ltd.