Search

Understanding the subduction and recycling of continental crust is crucial for reconstructing the long-term evolutionary history of Earth's mantle and crust. The Himalaya-Tibet orogen is arguably the world's best natural laboratory for investigating these processes. Cenozoic post-collisional ultrapotassic volcanic rocks are common in the Lhasa block of southern Tibet and they can provide important clues to crust-mantle interaction in a well-characterized continental collision zone. Understanding the sources and processes that generated these lavas can contribute to our understanding of the thermal and compositional characteristics of the deep mantle and geodynamic processes in this region, including Indian continental subduction. In this contribution, we report Sr-Nd-Pb-O-B isotope and elemental chemistry data for post-collisional (13-11 Ma) ultrapotassic rocks from the TangraYumco-Xuruco rift (TYXR) in the Lhasa block. The arc-like trace-element signatures and markedly enriched Sr-Nd-Pb-O isotope compositions indicate that these mafic rocks originate from a mantle source containing recycled crustal components. Unlike pre-collisional (∼64 Ma) ultrapotassic rocks in the Lhasa block with arc-like B/Nb (0.85-1.89) and δ 11 B (−9.0 to −2.5‰) values, the TYXR post-collisional ultrapotassic rocks with much lower B/Nb (0.05-0.85) and δ 11 B (down to −20.5‰) values resemble Miocene K-rich volcanic rocks from western Anatolia. These western Anatolian rocks have been formed by either progressive dehydration of a stalled slab or deep-subducted continental crust. However, some TYXR samples have lower B/Nb ratios than MORB, consistent with a fluid-starved source. These markedly negative δ 11 B in conjunction with low B/Nb cannot be explained by the addition of melts from oceanic sediments, which generally yield lower B/Nb but higher δ 11 B values than MORB (e.g., the Armenia post-collisional mafic rocks). Given the low δ 11 B of Indian upper continental crust and its similar Sr-Nd-Pb isotopic signatures to the post-collisional lavas, it is clear that the post-collisional ultrapotassic rocks in the Lhasa block contain a significant component derived from subducted Indian continental crust. Combined with the temporal evolution of regional magmatism, tectonics and geophysical data, we propose that the break-off and tearing of subducted Indian continental slab induced post-collisional magmatism in the Lhasa block. Our case study provides evidence that continental subduction contributes to very light B isotope compositions of post-collisional magmas, which suggests that B isotopes have the potential to discriminate between oceanic subduction and continental subduction. • Post-collisional ultrapotassic rocks in southern Tibet have very low B/Nb and δ 11 B. • B isotopes of these ultrapotassic rocks support contribution from Indian continental crust. • Melting of subducted Indian continental crust and interaction with overlying mantle generated these rocks. • B isotopes have the potential to discriminate between oceanic and continental subduction. [ABSTRACT FROM AUTHOR]

• Ba-Mg isotope data of mafic igneous rocks in a paleo-subduction zone are reported. • High δ 26 Mg values suggest subducted serpentinite components in their arc source. • Low δ 138 / 134 Ba values indicate contributions of subducted sediment. • A high δ 138 / 134 Ba value suggests contributions of subducted altered oceanic crust. • Arc magmas in paleo-subduction zones can be produced by mélange processes. In recent years, the mélange model has been increasingly considered as an important way to transfer slab components to arc sources in modern subduction zones. This model differs from the classic slab fluid/melt metasomatism model in that it invokes physical mixing of bulk sediment, altered oceanic crust (AOC), serpentinite, and mantle wedge at the slab-mantle interface. However, due to the lack of subducted sediment compositions, the mélange model has not been applied to any significant extent in paleo-subduction zones. The lack of evidence for bulk AOC and serpentinite components in mélange sources also hinders our understanding of any mélange origin for arc-type magmas. Here, we report Mg-Ba-Sr-Nd isotope compositions of the early Paleozoic Fushui mafic rocks in the Qinling orogen of central China, to trace their origin and constrain slab component transfer. The Fushui mafic rocks show typical arc-type geochemical features and enriched Sr-Nd isotope compositions, indicating the likely contribution of subducted sediments. They have low Rb/Ba ratios (<0.1), and most samples show δ 138 / 134 Ba values of −0.38 to +0.10‰, similar to those of sediments (−0.2 to +0.1‰) but lower than those of MORBs (+0.03 to +0.14‰), indicating that these low δ 138 / 134 Ba values are most likely derived from sediment components. One Fushui sample has a high δ 138 / 134 Ba of +0.31‰, similar to that of AOC (up to +0.4‰). This high δ 138 / 134 Ba is not correlated with fluid input; instead, it results from the contribution of bulk AOC. The Fushui rocks exhibit variable δ 26 Mg values (−0.23 to −0.11‰), slightly higher than those of MORBs. This most likely reflects 26Mg-enriched subducted serpentinite components in their source. Our results not only identify the variable slab components (sediment, AOC, and serpentinite) in the arc source, but also suggest that these slab components may be transferred to their arc source by the mélange process. This study therefore provides solid evidence for the generation of arc magmas by mélange processes in paleo-subduction zones, which confirms an important role for the mélange model in slab material transport. [ABSTRACT FROM AUTHOR]

• Two igneous pulses (ca. 290 Ma and 285 Ma) recognized for the Qiangtang-Panjal LIP. • The LIP was generated by a mantle plume and emplacement guided by slab pull forces. • The Neo-Tethys Ocean was initially opened at ∼285 Ma phase. Northward drift of the Cimmerian microcontinents and opening of the Neo-Tethys Ocean in Early Permian are prominent events in northern Gondwana's protracted breakup history. However, the geodynamic setting responsible for these events is controversial. The dispersal of the eastern Cimmerian microcontinents was accompanied by the emplacement of Qiangtang-Panjal large igneous province (QP-LIP). Previously published zircon U-Pb data imply the QP-LIP was emplaced over a long interval (320-280 Ma) but are inconsistent with the short eruption interval constrained by biostratigraphic studies. New secondary ion mass spectrometry (SIMS) zircon U-Pb ages from 15 mafic dikes and gabbroic intrusions from the Southern Qiangtang terrane as well as age data from other parts of the QP-LIP show a much shorter duration (∼ 290 − 285 Ma) for this LIP. The QP-LIP was characterized by an initial ∼290 Ma alkalic and tholeiitic phases and a subsequent ∼285 Ma tholeiitic phase, in agreement with regional biostratigraphic data constraining the timing of departure of the eastern Cimmerian microcontinents from Gondwana at ∼285 Ma and the opening of the Neo-Tethys Ocean. The QP-LIP was most likely the consequence of the arrival in the upper mantle of a mantle plume beneath the northern Gondwanan margin. This event, in combination with slab pull associated with subduction beneath Laurasia, may have caused extension and rifting of east Cimmerian continental fragments from the northern margin of Gondwana. This study may elucidate the mechanism responsible for repeated episodes of one-way northward drifting terranes from northern Gondwana that commenced in Paleozoic. [ABSTRACT FROM AUTHOR]

• We present the contemporary occurrence of both clinopyroxenes with Al-rich and -poor { h k 0 } sectors. • We propose a new crystal growth mechanism for clinopyroxene at high undercooling. • Al 2 O 3 -poor sectors are closer to equilibrium, rather than specific crystal faces. • Apatite inclusions in clinopyroxene can indicate rapid crystal growth. Recent experimental and natural studies have shown that igneous crystals can start to crystallize through skeletal or dendritic growth, and then backfill progressively to form polyhedral crystal morphologies, in contrast to the traditional tree-ring model of crystal growth. Identifying the growth mechanism of crystal is fundamental to understand crystal growth and associated magmatic processes. Clinopyroxene is an ubiquitous phase in magmatic systems. However, crystallographically controlled zoning patterns (e.g., sector zoning) observed in different studies are diverse or even show opposite trends, which make it challenging to understand the crystal growth mechanisms of clinopyroxene. This study presents a new finding of the contemporary occurrence of both clinopyroxenes with Al-rich and -poor { h k 0 } sectors in the Sailipu alkaline volcanic field in southern Tibet. Based on detailed textural and compositional analyses of a large number of clinopyroxene grains, we suggest a new crystal growth mechanism for clinopyroxene in mafic alkaline magmas. At low degrees of undercooling, all crystal faces grow concurrently, producing equilibrium Si–Mg-rich { − 111 } sectors and disequilibrium Al–Ti-rich { h k 0 } sectors; at high degrees of undercooling, the equilibrium sectors { h k 0 } initially form, which is followed by backfill of disequilibrium Al-rich { − 111 } sectors. Al 2 O 3 is a useful indicator of equilibrium, and Al 2 O 3 -poor sectors, rather than specific crystal faces, are closer to equilibrium. The behavior of other cations is complex and the distribution of their oxide components is variable. Textural observations and the modeling results for P transfer across the crystal–melt diffusive boundary layer show that apatite inclusions in clinopyroxene can indicate rapid crystal growth. These results may contribute to a better understanding of magma storage, ascent and eruption on the basis of clinopyroxene. [ABSTRACT FROM AUTHOR]