Hao-Yang Lee

Adakites from continental collision zones: Melting of thickened lower crust beneath southern Tibet

Adakites are geochemically distinct intermediate to felsic lavas found exclusively in subduction zones. Here we report the first example of such magmas from southern Tibet in an active continental collision environment. The Tibetan adakites were emplaced from ca. 26 to 10 Ma, and their overall geochemical characteristics suggest an origin by melting of eclogites and/or garnet amphibolites in the lower part (≥50 km) of thickened Tibetan crust. This lower-crustal melting required a significantly elevated geotherm, which we attribute to removal of the tectonically thickened lithospheric mantle in late Oligocene time. The identification of collision-type adakites from southern Tibet lends new constraints to not only the Himalayan-Tibetan orogenesis—how and when the Indian lithosphere started underthrusting Asia can be depicted—but also the growth of the early continental crust on Earth that consists dominantly of the tonalite-trondhjemite-granodiorite suites marked by adakitic geochemical affinities.

Miocene Jiali faulting and its implications for Tibetan tectonic evolution

The Karakoram^Jiali Fault Zone (KJFZ) comprises a series of right-lateral shear zones that southerly bound the eastward extrusion of northern Tibet relative to India and stable Eurasia. Here we present new 40 Ar/ 39 Ar age data from the Puqu and Parlung faults, two easternmost branches of the Jiali fault zone, which indicate a main phase of the KJFZ shearing from V18 to 12 Ma. Thus, the Tibetan eastward extrusion bounded by principal strike-slip fault zones started and was probably most active around the middle Miocene, an interval marked also by active east^west extension in southern Tibet. The coincidence of these two tectonic events strongly suggests a common causal mechanism, which is best explained as oblique convergence between India and Asia. Under the framework of this mechanism, the extension in southern Tibet is not a proxy for the plateau uplift. The KJFZ activity was furthermore coincident with right-lateral displacements along the Gaoligong and Sagaing faults in southeast Asia. This defines a Miocene deformation record for the regional dextral accommodation zone that, in response to the continuing India^ Asia collision, may have accounted for the initiation and prolonged history of clockwise rotation of the Tibetan extrusion around the eastern Himalayan Syntaxis. ? 2002 Elsevier Science B.V. All rights reserved.

Oldest Paleo-Tethyan ophiolitic mélange in the Tibetan Plateau

An early Paleozoic ophiolitic melange has recently been documented in the west Gangma Co area, north-central Tibetan Plateau. It is composed of serpentinite, isotropic and cumulate gabbros, basalt, and plagiogranite. Whole-rock geochemical data suggest that these rocks were formed in an oceanic-ridge setting, and the depletion in Nb and Ti indicates their suprasubduction zone nature. Furthermore, whole-rock e Nd ( t ) (+3.5 to +10.6) and zircon e Hf ( t ) values (+11.4 to +14.5) suggest that these rocks were derived from a long-term depleted mantle source. These geochemical features, combined with the rock types and their field relationships, suggest the rocks represent an ophiolite suite. Zircon U-Pb dating of isotropic gabbros and plagiogranites yielded weighted mean ages ranging from 437 to 501 Ma, which makes the west Gangma Co ophiolitic melange the oldest Paleo-Tethys ophiolitic melange in the Tibetan Plateau. The main Paleo–Tethys Ocean basin probably opened in the Middle Cambrian; it continued to grow throughout the Paleozoic and closed in the later Triassic. The Paleo–Tethys Ocean was formed by the breakup of the northern margin of Gondwana, with southward subduction of the proto-Tethys oceanic lithosphere along the northern margin of the supercontinent.

Old continental zircons from a young oceanic arc, eastern Taiwan: Implications for Luzon subduction initiation and Asian accretionary orogeny

Accretionary orogeny that involves terrain accretion and subsequent reprocessing plays a crucial role in the Phanerozoic tectonics and continental growth of Asia. This study reports zircon U-Pb and Hf isotope data from the Chimei igneous complex, eastern Taiwan, part of the intra-oceanic Luzon arc that has accreted onto the Eurasian continental margin since ca. 5 Ma. Among 292 U-Pb dates and 267 Hf isotope ratios obtained for zircon separates from six andesites, ten grains of magmatic zircons gave a mean 206 Pb/ 238 U age at 9.0 ± 0.3 Ma, with e Hf(t) values from +20 to +12, which we interpret as the emplacement age of the Chimei complex. Remaining zircons, however, show inherited ages clustering at ca. 14 Ma (n = 9) and ca. 220 Ma (n = 56, the largest peak), along with much older ages of ca. 0.7 Ga, 1.9 Ga, and 2.5 Ga. Whereas the ca. 14 Ma zircons may have crystallized from the earliest magmatism of the northern Luzon arc, the Indosinian and older zircons suggest Cathaysia-type sources that we attribute to a continental fragment that split off from the Eurasian margin by opening of the South China Sea and then drifted and accreted to the western Philippine Sea plate before the Luzon subduction initiation. Consequently, magmas derived from the depleted mantle wedge evolved and picked up the continental zircons during ascent. Our study not only better integrates regional tectonics with magmatic records in Southeast Asia, but also signifies a modern example from Taiwan that highlights the importance of ribbon continents in Asian orogenesis over time and space.

New U–Pb zircon ages of plagiogranites from the Nagaland–Manipur Ophiolites, Indo-Myanmar Orogenic Belt, NE India

We report new U–Pb zircon ages and whole-rock chemistry of plagiogranites from the Nagaland–Manipur Ophiolites, exposed in the Indo-Myanmar Orogenic Belt, NE India, which represents the southeastern extension of the Indus–Yarlung–Tsangpo Suture of Neo-Tethyan origin. The plagiogranites (SiO2 = 57 and 76 wt%) are characterized by high sodium (Na2O = 5.9 – 7.2 wt%) and low potassium (K2O = 0.1 – 0.6 wt%), coupled with low Rb (1.8 – 9.5 ppm), low Rb/Sr (<0.1) and A/CNK < 1 (molar). Geochemically they are oceanic plagiogranites with trondhjemitic compositions and metaluminous affinities. Their rare earth element (REE) patterns exhibit depletion in light REE (LaN/SmN = 0.50 – 1.13) relative to heavy REE (SmN/YbN = 0.59 – 0.88), with low ΣREE and incompatible trace element contents that indicate derivation from a depleted mantle source. High ratios of large ion lithophile elements to high field strength elements with pronounced Nb, Ta and Ti depletions furthermore suggest the presence of the subduction component in the source region. Zircon U–Pb geochronology yielded mean 206Pb/238U ages between 116.4 ± 2.2 and 118.8 ± 1.2 Ma that record crustal formation in the Early Cretaceous. Collectively the data indicate that the Nagaland–Manipur Ophiolites are broadly coeval and geochemically comparable with Neo-Tethyan ophiolites elsewhere in the Indus–Yarlung–Tsangpo Suture.

Significance of Zr-in-Rutile Thermometry for Deducing the Decompression P–T Path of a Garnet–Clinopyroxene Granulite in the Moldanubian Zone of the Bohemian Massif

This work aims to show the importance of Zr-in-rutile thermometry for evaluating the P–T history of granulite-facies rocks, where higher diffusion rates in the main constituent minerals impede the use of geothermometers based on element distributions. We apply Zr-in-rutile thermometry to a garnet–clinopyroxene (Grt–Cpx) granulite from the Moldanubian Zone of the Bohemian Massif. Three major metamorphic evolutionary stages are identified from the Grt–Cpx granulite. The early high-pressure (HP) stage is represented by an inclusion assemblage in garnet: a high-Ca garnet core (32–38% grossular, 30–32% pyrope and 32–35% almandine)þomphacite (36–39% jadeite and 3–5% Ca-Tschermak)þplagioclase (18% anorthite)þpargasitic amphiboleþ rutileþ zirconþquartz. The subsequent medium-pressure (MP) stage is represented by matrix minerals composed of augitic clinopyroxene (2–6% jadeite and 2–6% Ca-Tschermak)þorthopyroxeneþ ternary feldspar (17–23% anorthite, 41–44% albite, 33–43% orthoclase; reintegrated compositions from antiperthite grains in the matrix)þ rutileþ ilmeniteþquartz. The final low-pressure (LP) stage is represented by a symplectic corona composed of calcic plagioclase ( 90% anorthite)þorthopyroxeneþmagnetite. Application of Grt–Cpx and/or jadeite–quartz–albite geobarometers gives pressures of 1 8 GPa for the early HP stage and 1 3–1 4 GPa for the MP stage. The final LP stage is constrained to lower than 0 7 GPa using conventional geothermobarometers. Rutile inclusions in high-grossular garnet have a rather low and limited range of Zr contents (mostly 1100–1500 ppm), regardless of inclusion size. This suggests that rutile inclusions preserved the initial Zr compositions without much modification by later re-equilibration. Application of Zr-in-rutile thermometry yields a temperature of 830 C at 1 8 GPa for the early HP stage of granulite evolution. Rutile grains in undeformed clinopyroxene-rich domains of the matrix generally occur as small euhedral crystals and have higher Zr contents (mostly 8000–10000 ppm), corresponding to 980–1066 C at 1 35 GPa using Zr-in-rutile thermometry. In contrast, those in strongly deformed quartz-rich domains of the matrix occur as coarser and more elongated grains with lower Zr contents (3000– 5000 ppm), yielding slightly lower temperatures owing to retrogressive re-equilibration. Based on these results, we reveal that the studied Grt–Cpx granulite underwent a significant heating by about 200 C during the early stage of decompression from the peak pressure. Sensitive high-resolution ion microprobe U–Pb dating for the zircon inclusions in high-grossular garnet indicates that the HP stage of the studied granulite occurred at c. 340 Ma, which is indistinguishable from reported LP VC The Author 2017. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oup.com 1173 J O U R N A L O F P E T R O L O G Y Journal of Petrology, 2017, Vol. 58, No. 6, 1173–1198 doi: 10.1093/petrology/egx050 Advance Access Publication Date: 3 August 2017

Temporal and structural evolution of the Early Palæogene rocks of the Seychelles microcontinent

The Early Palæogene Silhouette/North Island volcano-plutonic complex was emplaced during the rifting of the Seychelles microcontinent from western India. The complex is thought to have been emplaced during magnetochron C28n. However, the magnetic polarities of the rocks are almost entirely reversed and inconsistent with a normal polarity. In this study we present new in situ zircon U/Pb geochronology of the different intrusive facies of the Silhouette/North Island complex in order to address the timing of emplacement and the apparent magnetic polarity dichotomy. The rocks from Silhouette yielded weighted mean 206Pb/238U ages from 62.4 ± 0.9 Ma to 63.1 ± 0.9 Ma whereas the rocks from North Island yielded slightly younger mean ages between 60.6 ± 0.7 Ma to 61.0 ± 0.8 Ma. The secular latitudinal variation from Silhouette to North Island is consistent with the anticlockwise rotation of the Seychelles microcontinent and the measured polarities. The rocks from Silhouette were emplaced across a polarity cycle (C26r-C27n-C27r) and the rocks from North Island were emplaced entirely within a magnetic reversal (C26r). Moreover, the rocks from North Island and those from the conjugate margin of India are contemporaneous and together mark the culmination of rift-related magmatism.

EARLY NEOPROTEROZOIC CRUST FORMATON IN THE DZABKHAN MICROCONTINENT, CENTRAL ASIAN OROGENIC BELT

The Dzabkhan microcontinent was defined by [Mossakovsky et al., 1994] as a cratonic terrane with an early Precambrian basement that combines highgrade metamorphic complexes of the Songino, Dzabkhan, Otgon, Baidarik, Ider and Jargalant Blocks. However, early Precambrian ages have so far only been recognized in the Baidarik and Ider blocks [Kozakov et al., 2007, 2011; Kroner et al., 2015].