Terrane

About: Terrane is a research topic. Over the lifetime, 11025 publications have been published within this topic receiving 442596 citations. The topic is also known as: tectonostratigraphic terrane.

Crustal evolution in north-east and east Africa from model Nd ages

Abstract: The relationships between the various crustal provinces that comprise north-east and east Africa are far from clear and reflect diverse tectonic and magmatic processes which took place during the Proterozoic. Here we present the results of an Nd isotope study on the major rock units of the Pan-African (1,100–500 Myr BP) terrane. Charnockites from Jabel Uweinat, a basement inlier at the junction of Egypt, Libya and the Sudan, yield middle Archaean model Nd ages, whilst model ages of <1,200 Myr have been obtained in a belt from the Eastern Desert of Egypt to north-west Kenya. This, therefore, represents a westward extension of the juvenile Pan-African crust of the Arabian Shield (Fig. 1). The shield itself is characterized by comparatively rapid crustal growth1, probably in an accreting island arc environment2. Between the two areas, there is a marginal zone of Pan-African rocks with relatively low ɛNd(T) values (see Table 1) due to increased contributions from the pre-Pan African crust. Overall, the Pan-African rocks from north-east and east Africa and those from the Damara of Namibia3 exhibit a wide range of ɛNd(T) from +7.5 to −18.0 which reflects regional changes in tectonic style and is not readily reconciled with simple models for the evolution of average continental crust.

Lower Cretaceous Strata in the Lhasa Terrane, Tibet, with Implications for Understanding the Early Tectonic History of the Tibetan Plateau

Abstract: Sedimentary strata in southern Tibet indicate that upper crustal deformation occurred throughout the region during Early Cretaceous time, suggesting that construction of the Tibetan plateau commenced tens of millions of years before the Late Cretaceous–early Tertiary Indo-Asian collision. Lower Cretaceous strata in the northern portion of the Lhasa terrane are characterized by lithic-rich conglomerate beds deposited in shallow marine and meandering-river fluvial environments. Sediments in these units were derived from two primary sources: volcanic rocks associated with Early Cretaceous intrusions, and sedimentary strata eroded from the northern Lhasa and southern Qiangtang terranes. The majority of detrital zircons from Lower Cretaceous fluvial conglomerate beds in northern Lhasa have U–Pb ages between 125 and 140 Ma and provide a maximum depositional age for these units of 125 ± 2 Ma. Lower Cretaceous strata in the southern portion of the Lhasa terrane consist of mudstone, quartzose sandstone, and subordinate quartzite-pebble conglomerate beds that were deposited in shallow marine and fluvial environments. Populations of detrital zircons in Lower Cretaceous conglomerate beds in southern Lhasa have U–Pb ages between 140 and 150 Ma, 500 and 600 Ma, and 850 and 950 Ma, and provide a maximum depositional age for these units of 143 ± 2 Ma. Both the modal composition and detrital-zircon U–Pb ages of the Lower Cretaceous conglomerate exposed in northern and southern Lhasa suggest different source areas, diachronous deposition, and possibly distinct genetic histories. Throughout most of the Lhasa terrane, the Lower Cretaceous clastic strata are overlain by a widespread limestone of Aptian–Albian age that was deposited in a shallow carbonate sea containing rudist patch reefs and muddy inter-reef zones. With respect to the tectono-sedimentary setting of the Lhasa terrane during Early Cretaceous time, the sedimentological and stratigraphic data are most consistent with a peripheral foreland basin model, which is interpreted to have resulted from the collision between the northern margin of the Lhasa terrane and the southern margin of Asia (the Qiangtang terrane). Several characteristics of the Aptian–Albian succession can be attributed to a peripheral foreland basin setting, although deposition within the region may have been influenced by a combination of mechanisms. Sedimentary characteristics of Lower Cretaceous rocks in the Lhasa terrane are consistent with recent ideas suggesting that portions of southern and central Tibet were deformed and above sea level before the Indo-Asian collision.

Rb-Sr isotope studies on Tinos Island (Cyclades, Greece); additional time constraints for metamorphism, extent of infiltration-controlled overprinting and deformational activity

Abstract: This study presents new Rb-Sr age data concerning the metamorphic evolution of the Attic- Cycladic Crystalline Belt which represents a complex polymetamorphic terrane within the Alpidic orogenic belt of the Hellenides. Two major groups of tectonic units can be distinguished. Metamorphism in parts of the upper units is commonly considered as a Cretaceous event. In contrast, the group of lower units experi- enced Tertiary high-pressure metamorphism which was followed by a medium-pressure overprint. We focus on the island of Tinos where a representative spectrum of the rock units found in the Cyclades is exposed in three tectonic units: the Upper Unit, the Intermediate Unit and the Basal Unit. The complete range of tectono-metamorphic and magmatic events affecting the Attic-Cycladic Crystalline Belt is documented by numerous petrological and tectonic studies. Phyllites and phyllonites from the ophiolitic Upper Unit yielded Rb-Sr apparent ages (phengite-whole-rock) between c. 92 and 21 Ma. The older age differs from the Cretaceous dates reported for upper unit rocks elsewhere in the Cyclades. It is suggested that the sequence studied belongs to the Jurassic ophiolites of the Hellenides rather than to Cretaceous occurrences. The spread to younger ages is related to non-pervasive rejuvenation and resetting of the Rb-Sr system during tectonic juxtaposition of the Upper Unit over the Intermediate Unit. The youngest age obtained so far for a sample from the Upper Unit (21 Ma) is believed to approximate the timing of tectonic juxtaposition which probably occurred during a regional greenschist-facies episode producing a pervasive overprint in the struc- turally lower tectonic unit. The major phyllite/meta-gabbro/serpentinite sequence of the Upper Unit is inter- preted as an emplacement-related ductile shear zone which experienced reworking under brittle conditions. In the Intermediate Unit, a gradient in Rb-Sr ages from top (c. 40 Ma) to the bottom (c. 22 Ma) was recog- nized, which is interpreted to represent greater effects of fluid infiltration and overprinting in the lower parts of this unit, possibly controlled by variable intensity of deformation which might be related to tectonic jux- taposition onto the Basal Unit. We suggest that synmetamorphic stacking of all three tectonic units took place during an Oligocene-Miocene greenschist event. Extensional deformation continued after tectonic stacking and after intrusion of the main granite, as is indicated by a Rb-Sr whole-rock isochron (15.1 ± 0.6 Ma) for a ductilely deformed garnet-bearing leucogranite from the marginal parts of the main undeformed pluton. Application of the Rb-Sr dating technique provided no unequivocal evidence that previously pub- lished Eocene K-Ar and 40 Ar- 39 Ar dates for high-pressure phengites from the lower units are significantly contaminated with excess argon.