Petrogenesis and tectonic implications of late Oligocene highly fractionated leucogranites in the Ailao Shan-Red River shear zone, SW China

Abstract Widespread anatexis was a regional response to the evolution of the Himalayan‐Tibetan Orogen that occurred some 30 Ma after collision between Asia and India. This paper reviews the nature, timing, duration and conditions of anatexis and leucogranite formation in the Greater Himalayan Sequence ( GHS ), and compares them to contemporaneous granites in the Karakoram mountains. Himalayan leucogranites and associated migmatites generally share a number of features along the length of the mountain front, such as similar timing and duration of magmatism, common source rocks and clockwise P–T paths. Despite commonalities, most papers emphasize deviations from this general pattern, indicating a fine‐tuned local response to the dominant evolution. There are significant differences in P–T– conditions during anatexis, and timing in relation to regional decompression. Further to that, some regions underwent a second event recording melting at low pressures. Zircon and monazite ages of anatectic rocks range between c . 25 and 15 Ma, suggesting prolonged crustal melting. Typically, a single sample may have ages covering most of this 10 Ma period, suggesting recycling of accessory phases from metamorphic rocks and early‐formed magmas. Recent studies linking monazite and zircon ages with their composition, have determined the timing of prograde melting and retrograde melt crystallization, thus constraining the duration of the anatectic cycle. In some areas, this cycle becomes younger down section, towards the leading front of the Himalayas, whereas the opposite is true in other areas . The relationship between granites and movement on the South Tibetan Detachment ( STD ) reveals that fault motion took place at different times and over different durations requiring complex internal strain distribution along the Himalayas. The nature and fate of magmas in the GHS contrast with those in the Karakoram mountains. GHS leucogranites have a strong crustal isotopic signature and migration is controlled by low‐angle foliation, leading to diffuse injection complexes concentrated below the STD . In contrast, the steep attitude of the Karakoram shear zone focused magma transfer, feeding the large Karakoram‐Baltoro batholith. Anatexis in the Karakoram involved a Cretaceous calcalkaline batholith that provided leucogranites with more juvenile isotopic signatures. The impact of melting on the evolution of the Himalayas has been widely debated. Melting has been used to explain subsequent decompression, or conversely, decompression has been used to explain melting. Weakening due to melting has also been used to support channel flow models for extrusion of the GHS , or alternatively, to suggest it triggered a change in its critical taper. In view of the variable nature of anatexis and of motion on the STD , it is likely that anatexis had only a second‐order effect in modulating strain distribution, with little effect on the general history of deformation. Thus, despite all kinds of local differences, strain distribution over time was such that it maintained the well‐defined arc that characterizes this orogen. This was likely the result of a self‐organized forward motion of the arc, controlled by the imposed convergence history and energy conservation, balancing accumulation of potential energy and dissipation, independent of the presence or absence of melt.

Anatexis

Leucogranite

Migmatite

Geochronology

10.1111/jmg.12204

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Citations (158)

Mantle inputs to Himalayan anatexis: Insights from petrogenesis of the Miocene Langkazi leucogranite and its dioritic enclaves

Lithos (2016)

Yuanchuan Zheng Zengqian Hou Qiang Fu Di‐Cheng Zhu Wei Liang

Leucogranite

Anatexis

Lithophile

Petrogenesis

Fractional crystallization (geology)

10.1016/j.lithos.2016.08.019

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Citations (67)

Decompression and anatexis of Himalayan metapelites

Tectonics (1994)

Nigel Harris John Massey

The Miocene leucogranites of the High Himalayas have been emplaced within a metasedimentary wedge, defined by the Main Central Thrust (MCT) and the South Tibetan Detachment System (STDS), a low‐angle detachment fault. Isotopic and tectonic constraints indicate that orogenic collapse along the South Tibetan Detachment System occurred at ∼20 Ma, synchronous with anatexis and emplacement of the leucogranites, thus suggesting that exhumation and anatexis were related. The isotope geochemistry of the Himalayan leucogranites indicates that their source lies within the metapelites of the metasedimentary wedge. Pelitic assemblages exhibit an inverted metamorphic geotherm consistent with wedge corner flow which stacked sillimanite‐grade thrust sheets onto kyanite‐grade rocks. However, the leucogranite protolith is not the sillimanite migmatites into which the melts have been emplaced but may be correlated with kyanite schists from near the base of the wedge. In the Langtang section of northern Nepal, leucogranite melts, formed from vapor absent incongruent melting of muscovite, were extracted from their source and migrated over distances >10 km before emplacement close to the STDS. A consequence of fractional melting in the hanging wall of the MCT is instability of the metasedimentary wedge. Decompression melting from depths >40 km can generate a melt fraction of ∼7%, depending on the initial temperature and muscovite content of the protolith. Enhancement of the available melt fraction during exhumation may have been critical in allowing the melt to migrate from its source under an extensional régime.

Leucogranite

Anatexis

Sillimanite

Protolith

Migmatite

Main Central Thrust

Metamorphic core complex

10.1029/94tc01611

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Citations (346)

Anatexis of late Neoarchean-Early Paleoproterozoic and late Paleoproterozoic garnet-bearing leucogranite from the Khondalite Belt in the North China Craton

International Geology Review (2022)

Qiang Shi Zhongyuan Xu Guochun Zhao Ding Ding Guangwei Liu

Two episodes of late Neoarchean-early Paleoproterozoic and late Paleoproterozoic garnet-bearing leucogranite, named the Baotou and Jining–Liangcheng garnet-bearing leucogranite are recognized from the Khondalite Belt in the North China Craton. Geochemical studies show that garnet-bearing leucogranite has high Al2O3 content and FeOT/MgO ratio, with a large variation in the CaO content and K2O/Na2O ratio. Additionally, the garnet-bearing leucogranite is enriched in light rare earth elements and large ion lithophile elements and depleted in Nb, Ta, P, and Ti. However, there are some variations in Eu anomalies, as indicated by the Eu-enriched and Eu-depleted patterns. The sensitive high-resolution ion microprobe (SHRIMP) zircon U-Pb dating of the Baotou garnet-bearing leucogranite and Jining–Liangcheng garnet-bearing leucogranite show that their anatectic ages were 2.35–2.43 Ga and 1.90–1.92 Ga, respectively, with zircon εHf (t) values of −0.01–3.93 and −4.36–1.99, and two-stage model ages (TDM2) of 2.76–3.02 Ga and 2.57–2.83 Ga, respectively. U-Th-Pb SHRIMP analyses on zircon reveal ages of ca. 2.35–2.47 Ga for the Baotou metapelite gneiss and ca. 1.90–1.91 Ga for the Jining–Liangcheng metapelite gneiss, which represent the ages of tectonic metamorphic events related to the anatexis. Our results, combined with geological and petrographical observations, geochemistry, and geochronology, suggest that (1) Garnet-bearing leucogranites formed as a result of anatexis of metapelite gneisses; (2) They are mixtures of leucosomes, residual minerals and mantle-derived materials; (3) Two episodes of garnet-bearing leucogranites have been identified; (4) Both two episodes of garnet-bearing leucogranites corresponded to late Neoarchean-early Paleoproterozoic and late Palaeoproterozoic tectonothermal events, respectively.

Leucogranite

Anatexis

10.1080/00206814.2022.2065545

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Citations (3)

Episodic crustal anatexis and the formation of Paiku composite leucogranitic pluton in the Malashan Gneiss Dome, Southern Tibet

中国科学通报：英文版 (2013)

Gao LiE Zeng LingSen Hou -

The Paiku composite leucogranitic pluton in the Malashan gneiss dome within the Tethyan Himalaya consists of tourmaline leucogranite,two-mica granite and garnet-bearing leucogranite.Zircon U-Pb dating yields that(1)tourmaline leucogranite formed at28.2±0.5 Ma and its source rock experienced simultaneous metamorphism and anatexis at 33.6±0.6 Ma;(2)two-mica granite formed at 19.8±0.5 Ma;(3)both types of leucogranite contain inherited zircon grains with an age peak at~480 Ma.These leucogranites show distinct geochemistry in major and trace elements as well as in Sr-Nd-Hf isotope compositions.As compared to the two-mica granites,the tourmaline ones have higher initial Sr and zircon Hf isotope compositions,indicating that they were derived from different source rocks combined with different melting reactions.Combined with available literature data,it is suggested that anatexis at~35 Ma along the Himalayan orogenic belt might have triggered the initial movement of the Southern Tibetan Detachment System(STDS),and led to the tectonic transition from compressive shortening to extension.Such a tectonic transition could be a dominant factor that initiates large scale decompressional melting of fertile high-grade metapelites along the Himalayan orogenic belt.Crustal anatexis at~28 Ma and~20 Ma represent large-scale melting reactions associated with the movement of the STDS.

Leucogranite

Anatexis

Tourmaline

Source

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Citations (0)

Crystallization and melting experiments of a fluorine-rich leucogranite from the Xianghualing Pluton, South China, at 150MPa and H2O-saturated conditions

Journal of Asian Earth Sciences (2002)

Xiaolin Xiong Bing Rao Fanrong Chen Zhu Jinchu Zhenhua Zhao

Leucogranite

Fractional crystallization (geology)

Liquidus