Regional geology of the Bangui Formation: insights offered by sedimentary geochemistry into the early evolution of an island arc

This work synthesizes the results of studies utilizing whole rock sedimentary geochemistry of samples collected from different tectonic settings. Major, trace and rare earth element diagrams effectively discriminate the nature of the source rocks and tectonic setting of the sedimentary rocks. The continental character of Palawan, Buruanga and Mindoro are affirmed by their geochemical components which point to felsic and quartzose recycled source rocks in a continental setting. The Antique samples, when plotted on the various diagrams, are shown to have contributions from both mafic and felsic sources. This indicates their deposition in an overlap basin which marks province linking resulting from the collision of the two blocks – Palawan Microcontinental Block and Philippine Mobile Belt. In stark contrast, the sedimentary rocks from Baguio (Klondyke, Amlang and Cataguintingan) have geochemical signatures consistent with their derivation from mafic igneous rocks in an oceanic island arc setting. The variety of diagrams used in this study illustrates their effectiveness in characterizing the provenance and tectonic setting of sedimentary rocks. These results underscore the value of using whole rock sedimentary geochemistry in deciphering the evolution of the Philippine island arc system.

Petrography and geochemistry of Cenozoic sedimentary sequences of the southern Samar Island, Philippines: Clues to the unroofing history of an ancient subduction zone

ABSTRACTThe use of geodynamic information contained in sedimentary rocks has only recently been extended into the tectonic reconstruction studies of the Philippine archipelago vis-à-vis the rest of the Southeast Asian region. We present here a comparative assessment of clastic units from the western Central Philippines, particularly from the islands of Mindoro, Panay and Palawan, and propose their likely association with sources of Cathaysian origin. Geochronological data from sedimentary formations in the study areas register U–Pb dating peaks at 185–140 Ma, 140–120 Ma and 112–90 Ma. These are similar to those observed of detrital zircons from rocks of Cathaysian origin in Taiwan and Southern China that chronicle the Yanshanian magmatic events. These same formations also record an older intercept at 1.9–1.85 Ga that likely corresponds to a regional continental orogenic episode recorded in the late Paleoproterozoic Cathaysian block. Major (e.g. Al2O3/TiO2) and trace-element (e.g. Y/Ni vs Cr/V) signatures of these sedimentary formations reflect stronger influences from granitic sources than mafic–ultramafic inputs that should otherwise be expected, considering their current oceanic island arc settings. Their La/Th and Th–Co–Zr/10 ratios also reveal continental island arc and active or passive continental margin depositional settings typical of rocks from the Palawan Microcontinental Block. New geochronological and geochemical data from the clastic rocks of northwest Mindoro, in addition to those already published for the other regions of the Palawan Microcontinental Block, provide further evidence for the amalgamation of fragments of Cathaysian origin within the Philippine island arc system.

The paper presents first high-precision data, U-Pb detrital zircon ages, whole-rock geochemistry, Hf-in-zircon and whole-rock Nd isotopes, from pre-Devonian (Ediacaran and Silurian) clastic sediments (sandstones) of the Tamdytau, Bukantau and Nuratau mountainous ranges of the Kyzylkum Desert and Nuratau Range in the western Uzbek Tienshan. The sediments form a turbidite-type complex associated with ocean plate stratigraphy units (oceanic pillow basalt, chert, siliceous mudstone and siltstone) and arc volcanic rocks. Four sandstone samples from the Tamdytau and Northern Nuratau Mts. (Besapan and Kaltadavan formations, respectively) yielded maximum depositional ages in the range of 570–540 Ma. These ages indicate the formation of pre-Devonian sedimentary units during a relatively narrow time interval from the latest Neoproterozoic (Ediacaran) to the earliest Cambrian, i.e. ca. 50–70 Myr. Five samples of turbidites from the Bukantau Mts. (Baimen Fm.) yielded a maximal depositional age of ca. 440 Ma, i.e. early Silurian (Llandovery). The petrographic, major and trace element compositions, as well as εNd(t) values ranging from -16 to -9 of those sandstones suggest their generally mature character and derivation from recycled orogens with a limited contribution of juvenile crust material. All sandstone samples yielded similar detrital zircon U-Pb age patterns characterized by major peaks at 650–570, 870–730, 1050–900 and 2400 Ma and by a smaller peak at ca. 1800 Ma. These patterns are similar to the U-Pb age spectra from the basement of the Tarim Craton. However, the Kyzylkum basement may contain a larger proportion of late Archaean rocks and igneous formations related to Ediacaran – earliest Cambrian orogenic events. All samples showing U-Pb detrital zircon age spectra with peaks at 650–570 Ma carry relatively large amounts of zircon grains with juvenile Hf isotope characteristics, i.e. positive εHf(t) (up to +10) suggesting their derivation from continental or Island arcs. The presence of exotic tectonic blocks composed of Ediacaran arc-type rocks hosted by the accretionary complex of the South Tienshan suggests that an extended arc system once existed at the southern convergent margin of the Turkestan Ocean. That arc system provided clastic material for the Palaeozoic sediments of the South Tienshan, but was probably destroyed by tectonic erosion during early Palaeozoic oceanic subduction.

Island arcs are postulated as the juvenile components that contribute to the growth of continental crust. Growth rates of arc crusts were previously computed using crustal thicknesses derived from seismic data. Consequently, crustal growth rates of oceanic island arcs are also constrained by the limited seismic data availability. This work presents the first comparison of gravity-derived magmatic growth rates of Western Pacific oceanic island arcs. We used the statistical correlation between Bouguer anomalies and seismic-derived crustal thicknesses to generate an empirical formula. The new empirical formula was utilized to estimate the crustal thicknesses of oceanic island arcs using Bouguer anomalies from the EGM2008 global gravity model. The resulting crustal thicknesses were employed to compute the magmatic growth rates of western Pacific island arcs and the Philippine island arc system. The latest magmatic growth rate estimates show that the magmatic productivity of Western Pacific island arcs, which are directly associated with Pacific Plate subduction, is significantly higher (28–60 km3/km/m.y). The growth rate of the Pacific island arcs is higher compared to the magmatic growth rate computed for the other oceanic island arcs (12–25 km3/km/m.y), which are derived from the subduction of other oceanic lithospheres (i.e., the Philippine Sea Plate; Caribbean Sea Plate; and Eurasia-South China Sea slabs). This is attributed to the variation in the ages of the subducting plates. The Pacific Plate, being older, is associated with higher degrees of serpentinization and sediment cover, which introduce more volatiles inducing more robust partial melting of the mantle wedge.

Mid-ocean ridge basalt (hereafter, MORB) is a final product of melt generated from the partial melting of mantle peridotite, following reaction with mantle and/or lower crustral rocks, fractionation at a shallower crust and other processes en route to seafloor. Therefore, it is difficult to estimate melting processes at the upper mantle solely from any investigations of MORB. In contrast to the restricted occurrence of peridotite of mantle origin in particular tectonic settings (e.g., ophiolites, fracture zones, or oceanic core complexes), the ubiquitous presence of MORB provides us with a key to understanding global geochemical variations of the Earth's interior in relation to plate tectonics. In fact, MORB has been considered to show a homogeneous chemical composition. In terms of volcanic rocks from other tectonic settings (e.g., island arc, continental crust, ocean island), this simple concept seems to be true. However, recent investigations reveal that even MORB has significant chemical variations that seem to correspond to location (Pacific, Atlantic, and Indian Oceans). These observations suggest that the mantle beneath each ocean has a distinct chemical composition and an internally heterogeneous composition. In this paper, global geochemical variations of MORB in terms of major and trace element compositions and isotope ratios are examined using a recently compiled database. The compilation suggests that MORB has heterogeneous compositions, which seem to originate from a mixture of depleted mantle and some enriched materials. Coupled with trace element compositions and Pb-isotope ratios, there seems to be at least two geochemical and isotopic domain of the upper most mantle: equatorial Atlantic-Pacific Oceans and southern Atlantic-Indian Ocean. Material (melt and/or solid) derived from plume, subducted slab, subcontinental crust, or fluid added beneath an ancient subduction zone is a candidate to explain the enrichment end-member to produce heterogeneous MORB. Because MORB is heterogeneous, using a tectonic discrimination diagram that implicitly subsumes homogeneous MORB or its mantle sources should be reconsidered. Further investigations, particularly of off-axis MORB, are needed to understand the relationship between heterogeneous compositions of MORB and geophysical parameters (e.g., degree of melting, temperature, spreading rate, crustal thickness, etc). In addition, the role of the MOHO transitional zone should be investigated to interpret the chemical characteristics of MORB.

Zircon U–Pb and Lu–Hf isotopic and geochemical constraints on the origin of the paragneisses from the Jiaobei terrane, North China Craton

Development of the Philippine Mobile Belt in northern Luzon from Eocene to Pliocene

The influence of flood basaltic source terrains on the efficiency of tectonic setting discrimination diagrams: An example from the Gulf of Khambhat, western India

Geochemistry, provenance, and tectonic setting of Neoproterozoic metavolcanic and metasedimentary units, Werri area, Northern Ethiopia

Early Cretaceous adakite from the Atlas porphyry Cu-Au deposit in Cebu Island, Central Philippines: Partial melting of subducted oceanic crust

Early Cretaceous arc volcanic suite in Cebu Island, Central Philippines and its implications on paleo-Pacific plate subduction: Constraints from geochemistry, zircon U–Pb geochronology and Lu–Hf isotopes

Geodynamic evolution of the Baguio Mineral District: Unlocking the Cenozoic record from clastic rocks

An island arc origin of plagiogranites at Oytag, western Kunlun orogen, northwest China: SHRIMP zircon U–Pb chronology, elemental and Sr–Nd–Hf isotopic geochemistry and Paleozoic tectonic implications

The Pleistocene (2.2–1.5 Ma) Koloula Igneous Complex (KIC) on Guadalcanal in the Solomon island arc consists of a low-K calc-alkaline sequence of ultramafic to felsic plutonic rocks. We present whole-rock major and trace element and Sr–Nd-Pb isotope data, as well as mineral compositions that record the magmatic evolution of the complex. The intrusive sequence is grouped into two cycles, Cycle 1 and 2, comprising gabbroic or dioritic to granodioritic rocks. The major and trace element data of each cycle forms a single calc-alkaline fractional crystallisation trend. The distinct radiogenic isotope and incompatible element compositions of the Cycle 1 and 2 intrusions imply slightly different mantle sources. The KIC formed by shallow (0.1 GPa) fractional crystallisation of mantle-derived Al-rich basaltic parental magmas (6–8 wt.% MgO) that were formed by deeper-level (0.7 GPa) fractionation of olivine and pyroxene from Mg-rich (~ 11 wt.% MgO) primary magmas in the Solomon intra-oceanic island arc. Olivine, clinopyroxene, plagioclase, amphibole, biotite, apatite, and Fe–Ti oxides fractionated from the KIC’s high-Al basaltic parental magmas to form calc-alkaline magmas. Liquid line of descent trends calculated using mass balance calculations closely match major element trends observed in the KIC data. The KIC crystallised at shallow, upper crustal depths of ~ 2.0–3.0 km in ~ 20 km-thick island arc crust. This complex is typical of other Cenozoic calc-alkaline ultramafic to felsic plutons in Pacific intra-oceanic island arcs in terms of field relationships, petrology, mineral chemistry and whole-rock geochemistry. Hornblende fractionation played a significant role in the formation of the calc-alkaline felsic plutonic rocks in these Cenozoic arc plutons, causing an enrichment of SiO2 and light rare earth elements. These plutons represent the fossil magma systems of arc volcanoes; thus, the upper arc crust is probably generated by migration of magmatic centres.