Late Magmatic Rocks Research Articles

Early Mesozoic Magmatism Within the Tibetan Plateau: Implications for the Paleo‐Tethyan Tectonic Evolution and Continental Amalgamation

AbstractNumerous early Mesozoic magmatic rocks within the Tibetan Plateau ultimately have a geodynamic controll caused by the Paleo‐Tethyan tectonic evolution and the assembly of Pangea. Based on their temporal variation and the timing of closure (235 ± 10 Ma) of the Paleo‐Tethys Ocean, they can be broadly divided into two groups by zircon U‐Pb geochronology: an Early to Middle Triassic (252–235 Ma) group and a Late Triassic to Early Jurassic (ca. 235–190 Ma) group. Early magmatic rocks are mainly found in the middle northern Tibetan Plateau, whereas late magmatic rocks are more widespread in the whole Tibetan Plateau. Nd and Hf isotope mapping suggests that the eastern segment of the Qiangtang is underlain by old (late Archean to early Paleoproterozoic) basement, while Southern Lhasa is underlain by juvenile lower crust, which yields Neoproterozoic to early Paleozoic model ages. Early Mesozoic tectonomagmatism was likely caused by episodic southward opening of multi‐Tethys oceans, punctuated by episodes of subduction and collision. The Middle to Late Permian tectonomagmatic history of Paleo‐Tethys was dominated by continental arc‐related systems. During the Early to Middle Triassic, the tectonomagmatic history was associated with the ongoing closure of the Paleo‐Tethys Ocean and a tectonic transition from subduction to collision. Late Triassic tectonomagmatism formed in a post‐collisional setting following the closure of the Paleo‐Tethys Ocean and the ultimate assembly of Pangea. Early Mesozoic magmatism within the Tibetan Plateau thus provides crucial information on the history of oceanic‐continental evolution and helps constrain the tectonic transition from subduction/accretion through to collision/postcollision for East Paleo‐Tethys.

Hualong Complex, South Qilian terrane: U–Pb and Lu–Hf constraints on Neoproterozoic micro-continental fragments accreted to the northern Proto-Tethyan margin

The Neoproterozoic Hualong Complex dominates the South Qilian terrane. It contains mafic to felsic magmatic rocks, cherts and turbiditic sediments, some of which have been metamorphosed to amphibolite grade and is traditionally regarded as a pre-Cambrian micro-continental block rifted from the South China plate. New U–Pb geochronology of morphologically complex zircons sheds light on the history of the complex. Garnet-bearing amphibolite and quartzite have detrital zircon U–Pb main peak ages of 1.47–1.78Ga with youngest U–Pb ages of 967Ma and 964Ma. They are intruded by Neoproterozoic orthogneisses that crystallized between ca. 850 and 940Ma. Psammitic paragneisses are dominated by 940–780Ma detrital zircon populations with a youngest U–Pb weighted mean age of 721±3Ma and a main peak at 906Ma.Metamorphic overgrowths on older zircons record amphibolite facies metamorphism of the Hualong Complex and correspond to the emplacement of the younger plutonic rocks during the Early Paleozoic northward subduction of Proto-Tethyan oceanic lithosphere. 426Ma and 455Ma detrital zircon populations that originated from synkinematic granitoids dominate quartz–mica schist.Negative to positive ɛHf(t) values for both 940–850Ma granitic orthogneisses and intruding 460–410Ma granites suggest a crustal–mantle mixture source. Hf TDM1 model ages and inherited zircon U–Pb ages are the same as those for garnet-bearing amphibolites and psammitic gneiss indicating that late magmatic rocks were formed by melting of the accreted Neoproterozoic rocks.

The thermal history of the Eastern Ghats Belt (India) as revealed by U–Pb and [formula omitted] dating of metamorphic and magmatic minerals: implications for the SWEAT correlation

The Eastern Ghats belt is a large high-grade terrane exposed along the east coast of India. The late Proterozoic orogen consists of N–S trending charnockite and metasedimentary belts. The younger E–W trending Godavari Rift divides the orogen into a northern and a southern segment. Textural and structural relationships indicate a complex thermo-tectonic evolution involving several episodes of metamorphism and deformation. This orogenic belt represents an important part of the reconstructed global Southwest-United-States–East-Antarctica (SWEAT) orogen. The exact timing of metamorphism and tectonism in this large orogenic belt plays a key role in this global reconstruction. Textural and structural relationships indicate a complex thermo-tectonic evolution involving several episodes of metamorphism and deformation. In order to reconstruct parts of the regional thermal history of the Eastern Ghats belt, U–Pb ages were determined on monazite, allanite, zircon and sphene and 40Ar/ 39Ar ages on hornblende. Monazite, allanite and some sphenes provide evidence that in the central and eastern tectonic units north of the Godavari Rift the last high-grade metamorphism occurred at ca. 960 Ma. In the most western unit, the Western charnockite zone, allanite and monazite from late pegmatites indicate a major thermal event around 1.6 Ga. This is the first indication that there may be pronounced temporal differences in the thermal evolution of different tectonic or ltihological units in the Eastern Ghats belt. The majority of the sphenes from the Eastern Ghats belt are discordant and lie along a reference line extending from ca. 935 Ma to 504 Ma. This discordance is interpreted to be due to a thermal disturbance during a Pan-African deformation phase, which is dated by the emplacement of apatite–magnetite veins that contain zircon with a concordant U–Pb age of 516±1 Ma. All hornblende 40Ar/ 39Ar ages from the central units of the belt provide Pan-African plateau ages. The partially reset sphene and completely reset hornblende ages indicate that the Pan-African metamorphism reached at least middle-amphibolite facies conditions in parts of the Eastern Ghats belt. This thermal overprint was not strong enough to cause retrogression of the high-grade mineral assemblages, but has so far only been recognized in the reset mineral ages and some late magmatic rocks. A hornblende 40Ar/ 39Ar age of ca. 1110 Ma from an amphibolite south of the Godavari rift indicates that the Pan-African thermal overprint was weaker in the Western charnockite zone, if it affected this area at all. The new ages presented for the Eastern Ghats belt are similar to published ages for the Rayner Complex and the Prydz Bay region of Eastern Antarctica. This similarity in ages is strong support for these areas in East Antarctica and the Eastern Ghats belt being complementary parts of an extensive orogenic belt formed during the Grenvillian Orogeny around 1 Ga. Both continental fragments also show evidence for a Pan-African overprint, however, this event is much more pronounced in Antarctica than in the Eastern Ghats belt.