Sub-arc mantle enrichment in the Sunda rear-arc inferred from HFSE systematics in high-K lavas from Java

Many terrestrial silicate reservoirs display a characteristic depletion in Nb, which has been explained in some studies by the presence of reservoirs on Earth with superchondritic Nb/Ta. As one classical example, K-rich lavas from the Sunda rear-arc, Indonesia, have been invoked to tap such a high-Nb/Ta reservoir. To elucidate the petrogenetic processes active beneath the Java rear-arc and the causes for the superchondritic Nb/Ta in some of these lavas, we studied samples from the somewhat enigmatic Javanese rear-arc volcano Muria, which allow conclusions regarding the across-arc variations in volcanic output, source mineralogy and subduction components. We additionally report some data for an along-arc sequence of lavas from the Indonesian part of the Sunda arc, extending from Krakatoa in the west to the islands of Bali and Lombok in the east. We present major and trace element concentrations, Sr–Nd–Hf–Pb isotope compositions, and high-field-strength element (HFSE: Nb, Ta, Zr, Hf, W) concentrations obtained via isotope dilution and MC-ICP-MS analyses. The geochemical data are complemented by melting models covering different source compositions with slab melts formed at variable P–T conditions. The radiogenic isotope compositions of the frontal arc lavas in combination with their trace element systematics confirm previously established regional variations of subduction components along the arc. Melting models show a clear contribution of a sediment-derived component to the HFSE budget of the frontal arc lavas, particularly affecting Zr–Hf and W. In contrast, the K-rich rear-arc lavas tap more hybrid and enriched mantle sources. The HFSE budget of the rear-arc lavas is in particular characterized by superchondritic Nb/Ta (up to 25) that are attributed to deep melting involving overprint by slab melts formed from an enriched garnet–rutile-bearing eclogitic residue. Sub-arc slab melting was potentially triggered along a slab tear beneath the Sunda arc, which is the result of the forced subduction of an oceanic basement relief ~ 8 Myr ago as confirmed by geophysical studies. The purported age of the slab tear coincides with a paucity in arc volcanism, widespread thrusting of the Javanese basement crust as well as the short-lived nature of the K-rich rear-arc volcanism at that time.

Lava from Bali Volcanoes Offers Window into Earth’s Mantle

Lava from the Agung and Batur volcanoes provides a near-pristine picture of Earth’s mantle and raises questions about all volcanoes along the Indonesian Sunda Arc and beyond.

Sunda arc mantle source δ18O value revealed by intracrystal isotope analysis

Magma plumbing systems underlying subduction zone volcanoes extend from the mantle through the overlying crust and facilitate protracted fractional crystallisation, assimilation, and mixing, which frequently obscures a clear view of mantle source compositions. In order to see through this crustal noise, we present intracrystal Secondary Ion Mass Spectrometry (SIMS) δ18O values in clinopyroxene from Merapi, Kelut, Batur, and Agung volcanoes in the Sunda arc, Indonesia, under which the thickness of the crust decreases from ca. 30 km at Merapi to ≤20 km at Agung. Here we show that mean clinopyroxene δ18O values decrease concomitantly with crustal thickness and that lavas from Agung possess mantle-like He-Sr-Nd-Pb isotope ratios and clinopyroxene mean equilibrium melt δ18O values of 5.7 ‰ (±0.2 1 SD) indistinguishable from the δ18O range for Mid Ocean Ridge Basalt (MORB). The oxygen isotope composition of the mantle underlying the East Sunda Arc is therefore largely unaffected by subduction-driven metasomatism and may thus represent a sediment-poor arc end-member.

The Eocene−Oligocene Transition in Nanggulan, Java: lithostratigraphy, biostratigraphy and foraminiferal stable isotopes

The Nanggulan section in south central Java comprises open marine sediments and volcanic deposits of Eocene-Oligocene age that accumulated in a marginal basin within the young Sunda Arc complex. A new borehole captures the stratigraphy and showcases the exceptional preservation of calcareous microfossils across an apparently complete Eocene-Oligocene Transition (EOT), a time interval significant for the initiation of continental scale glaciation on Antarctica. Low-resolution benthic and planktonic foraminifera oxygen and carbon stable isotopes (δ18O and δ13C) record increasing δ18O and δ13C in the basal Oligocene, allowing correlation to global records. Isotopic values imply warm temperatures and relatively high nutrients along the SE Java margin. The Nanggulan EOT is a valuable archive for reconstructing ocean-climate behavior and plankton evolution and extinction in the Indo-Pacific warm pool. The borehole also adds to understanding of the early stages of Sunda Arc volcanism.Supplementary material:https://doi.org/10.6084/m9.figshare.c.5429453

Middle Jurassic arc reversal, Victoria–Katha Block and Sibumasu Terrane collision, jadeite formation and Western Tin Belt generation, Myanmar

Myanmar is occupied by the N-wards continuation of the Sunda arc and by the Shan Plateau and its continuation through Yunnan into Tibet. Our new tectonic interpretation of the ophiolite–flysch belts, world-famous jadeite and tin deposits in Myanmar west of the Salween adopts previous proposals that, before 450-km post-early Oligocene dextral displacement along the Sagaing Fault, the ophiolite belt in NE Myanmar continued through the topography that is now located west of the fault in the Indo-Burman Ranges. Differences in cross-section through Mogok and the Shan Scarps are reconciled by the recently proposed emplacement, in our view during Permian time, of the Mogok Metamorphic Group onto the Slate Belt to form Sibumasu. We argue that during Early Jurassic time a Neo-Tethys ophiolite nappe was obducted over turbidites on Sibumasu’s passive western margin. Following reversal in tectonic polarity, the remaining Neo-Tethys subducted E-wards generating the 113–128 Ma Mondaung Arc. During ocean closure the Victoria–Katha Block and its Triassic flysch subducted beneath Sibumasu, resulting in jadeite veins in overlying serpentinite that ascended in the subduction zone and were exhumed at Hpakant and Nat Hmaw, bordering the Jade Mines Uplift. Subduction of the Indian Ocean since Albian time generated the Popa–Loimye arc, while extensional faulting led to uplift of the Indo-Burman Ranges and to the formation of the Western Tin Belt granites. Tectonic effects in Myanmar of the India–Asia collision may be confined to the Disang thrust belt in the Naga Hills.

Geology, Rock Geochemistry and Ore Fluid Characteristics of the Brambang Copper-Gold Porphyry Prospect, Lombok Island, Indonesia.

Brambang is one of the porphyry copper-gold prospects/deposits situated along eastern Sunda arc. This study is aimed to understand geological framework, alteration geochemistry and ore fluid characteristics of the prospect. Fieldworks and various laboratory analyses were performed including petrography, ore microscopy, rock geochemistry, chlorite chemistry and fluid inclusion microthermometry. The prospect is composed of andesitic tuff and diorite which are intruded by tonalite porphyries. Tonalite porphyries are interpreted as ore mineralisation-bearing intrusion. Various hydrothermal alterations are identified including potassic, phyllic, propylitic, advanced argillic and argillic types. Ore mineralisation is characterized by magnetite and copper sulfides such as bornite and chalcopyrite. Potassic alteration is typified by secondary biotite, and associated with ore mineralisation. Mass balance calculation indicates SiO2, Fe2O3, K2O, Cu and Au are added during potassic alteration process. Ore forming fluid is dominated by magmatic fluid at high temperature (450-600ºC) and high salinity (60-70 wt. % NaCl eq.). Hydrothermal fluid was diluted by meteoric water incursion at low-moderate temperature of 150-400ºC and salinity of 0.5-7 wt. % NaCl eq.

Mid-Miocene volcanic migration in the westernmost Sunda arc induced by India-Eurasia collision

The migration of arc magmatism that is a fundamental aspect of plate tectonics may reflect the complex interaction between subduction zone processes and regional tectonics. Here we report new observations on volcanic migration from northwestern Sumatra, in the westernmost Sunda arc, characterized by an oblique convergent boundary between the Indo-Australian and Eurasian plates. Our study indicates that in northwestern Sumatra, volcanism ceased at 15–10 Ma on the southern coast and reignited to form a suite of active volcanoes that erupt exclusively to the north of the trench-parallel Sumatran fault. Younger volcanic rocks from the north are markedly more enriched in K2O and other highly incompatible elements, delineating a geochemical variation over space and time similar to that in Java and reflecting an increase in the Benioff zone depth. We relate this mid-Miocene volcanic migration in northwestern Sumatra to the far-field effect of propagating extrusion tectonics driven by the India-Eurasia collision. The extrusion caused regional deformation southward through Myanmar to northwestern Sumatra and thus transformed the oblique subduction into a dextral motion–governed plate boundary. This tectonic transformation, associated with opening of the Andaman Sea, is suggested to be responsible for the volcanic migration in northwestern Sumatra.

Deformation, earthquakes and tsunamis along thickly sedimented subduction: Arakan segment of the Sunda Arc

<p>Incoming sediment thickness and composition are primary factors in the morphology and shallow structure of subduction boundaries. Sediment thickness in the Indian Ocean increases SE to NW along the Sunda arc. From <1km along Java to >15km where the boundary encounters the Ganges-Brahmaputra Delta (GBD). Here the accretionary prism broadens to the NW to >300 km wide. It is dominated by shallow-water to non-marine sediment. This segment also features a broad shallow megathrust overlain by linear anticlines rooted in splay faults. It is entirely above sea level and blind in its frontal part. This GBD segment transitions to a more familiar subduction structure and morphology along the submerged Arakan segment to the SE. The SE portion of this segment is characterized by larger splay faults that expose deep-water sediment with mud diapirism forming volcanoes and circular synclines. With increasing sediment input, the NW portion of the Arakan segment encroaches onto the GBD shelf. Both the SE and NW portions of the Arakan segment ruptured in the Mw>8.5 1762 tsunamigenic earthquake according to field and modeling evidence.</p><p>Uplifted coral reefs and marine terraces along the Myanmar and Bangladesh coasts document a >500 km rupture in 1762. The uplift, ranging from 6 m to 2 m from south to north, has been linked to rupture on the megathrust and on shallow splays. Tsunami deposits are traced for ~10 km along the St. Martin’s Island anticline and for >40 km along the Teknaf peninsula. Microfossils and mollusk assemblages in these deposits are consistently of shallow water affinity and date the tsunami to 1762. This deposit covers only a small fraction of the inferred megathrust rupture. If it is representative of the total tsunami distribution, a local anticline may have been the main source. Evidence from live coral microatolls show uplift on St. Martin’s Island continuing 250 years after the earthquake. This motion could stem from continued anelastic deformation of the anticline updip of the rupture. More widely distributed evidence from sediment and corals could address questions about megathrust and splay behavior in 1762 and after. Plans include multichannel seismic surveying, high resolution subbottom profiling and 40 m long piston coring to compare the SE to NW shelf portions to the Arakan segment along the Myanmar and Bangladesh coasts. More generally, we aim to better understand subduction and geohazards along thickly sedimented systems.</p>

Supplemental Material: Mid-Miocene volcanic migration in the westernmost Sunda arc induced by India-Eurasia collision

Analytical methods, ages and geochemical data.<br>

Supplemental Material: Mid-Miocene volcanic migration in the westernmost Sunda arc induced by India-Eurasia collision

Analytical methods, ages and geochemical data.<br>