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.

Eocene to Miocene tectonostratigraphic evolution of northwest New Zealand

<p>Reinga Basin is located northwest of New Zealand, along strike structurally from Northland and has a surface area of ~150,000 km². The basin contains deformed Cretaceous and Cenozoic strata, flat unconformities interpreted as sea level-modulated erosion surfaces and is intruded by volcanics. Persistent submarine conditions and moderate water depths has led to preservation of fossil-rich bathyal sedimentary records. This thesis presents the first seismic-stratigraphic analysis tied to dredged rock samples and recent International Ocean Discovery Program (IODP) drilling. The Cenozoic tectonic evolution of Reinga Basin comprises four main phases. (1) Folding and uplift from lower bathyal water depths occurred at 56-43 Ma along West Norfolk Ridge to produce wave ravinement surfaces. This phase of deformation in Reinga Basin pre-dates tectonic events onshore New Zealand. (2) Basin-wide 39-34 Ma compression and reverse faulting exposed early to middle Eocene strata at the seabed. This phase of deformation is also observed farther south in Taranaki. (3) Oligocene uplift is recorded by late Oligocene shallow-water fauna at Site U1508, and led to a 6 Myr hiatus (34-28 Ma) associated with flat wave ravinement surfaces nearby. The unconformity is temporally associated with: normal faulting near West Norfolk Ridge that created topography of Wanganella Ridge; onset of Reinga Basin volcanism; and emplacement of South Maria Allochthon. Thin-skinned deformation and volcanism post-date thick-skinned reverse faulting and folding. The end of reverse faulting near South Maria Ridge is determined from undeformed Oligocene strata that have subsided 1500-2000 m since 36-30 Ma. (4) During the final phase of Reinga Basin deformation, South Maria Ridge subsided ~900-1900 m from middle shelf to bathyal depths from 23-19 Ma. Deformation migrated southeastwards, culminating in Northland Allochthon emplacement (23-20 Ma) and onshore arc volcanism at 23-12 Ma. Eocene onset of tectonic activity in northern New Zealand is shown to be older than previously recognised and it was broadly synchronous with other events related to subduction initiation and plate motion change elsewhere in the western Pacific.</p>

Eocene to Miocene tectonostratigraphic evolution of northwest New Zealand

<p>Reinga Basin is located northwest of New Zealand, along strike structurally from Northland and has a surface area of ~150,000 km². The basin contains deformed Cretaceous and Cenozoic strata, flat unconformities interpreted as sea level-modulated erosion surfaces and is intruded by volcanics. Persistent submarine conditions and moderate water depths has led to preservation of fossil-rich bathyal sedimentary records. This thesis presents the first seismic-stratigraphic analysis tied to dredged rock samples and recent International Ocean Discovery Program (IODP) drilling. The Cenozoic tectonic evolution of Reinga Basin comprises four main phases. (1) Folding and uplift from lower bathyal water depths occurred at 56-43 Ma along West Norfolk Ridge to produce wave ravinement surfaces. This phase of deformation in Reinga Basin pre-dates tectonic events onshore New Zealand. (2) Basin-wide 39-34 Ma compression and reverse faulting exposed early to middle Eocene strata at the seabed. This phase of deformation is also observed farther south in Taranaki. (3) Oligocene uplift is recorded by late Oligocene shallow-water fauna at Site U1508, and led to a 6 Myr hiatus (34-28 Ma) associated with flat wave ravinement surfaces nearby. The unconformity is temporally associated with: normal faulting near West Norfolk Ridge that created topography of Wanganella Ridge; onset of Reinga Basin volcanism; and emplacement of South Maria Allochthon. Thin-skinned deformation and volcanism post-date thick-skinned reverse faulting and folding. The end of reverse faulting near South Maria Ridge is determined from undeformed Oligocene strata that have subsided 1500-2000 m since 36-30 Ma. (4) During the final phase of Reinga Basin deformation, South Maria Ridge subsided ~900-1900 m from middle shelf to bathyal depths from 23-19 Ma. Deformation migrated southeastwards, culminating in Northland Allochthon emplacement (23-20 Ma) and onshore arc volcanism at 23-12 Ma. Eocene onset of tectonic activity in northern New Zealand is shown to be older than previously recognised and it was broadly synchronous with other events related to subduction initiation and plate motion change elsewhere in the western Pacific.</p>

Age, chemistry, and tectonic setting of Miramichi terrane (Early Paleozoic) volcanic rocks, eastern and east-central Maine, USA

Volcanic rocks in the Miramichi inlier in Maine occur in two areas separated by the Bottle Lake plutonic complex: the Danforth segment (Stetson Mountain Formation) north of the complex and Greenfield segment to the south (Olamon Stream Formation). Both suites are dominantly pyroclastic, with abundant andesite, dacite, and rhyolite tuffs and subordinate lavas, breccias, and agglomerates. Rare basaltic tuffs and a small area of basaltic tuffs, agglomerates, and lavas are restricted to the Greenfield segment. U–Pb zircon geochronology dates Greenfield segment volcanism at ca. 469 Ma, the Floian–Dapingian boundary between the Lower and Middle Ordovician. Chemical analyses reveal a calc-alkaline suite erupted in a continental volcanic arc, either the Meductic or earliest Balmoral phase of Popelogan arc activity. The Maine Miramichi volcanic rocks are most likely correlative with the Meductic Group volcanic suite in west-central New Brunswick. Orogen-parallel lithologic and chemical variations from New Brunswick to east-central Maine may result from eruptions at different volcanic centers. The bimodal Poplar Mountain volcanic suite at the Maine–New Brunswick border is 10–20 myr younger than the Miramichi volcanic rocks and more likely an early phase of back-arc basin rifting than a late-stage Meductic phase event. Coeval calc-alkaline arc volcanism in the Miramichi, Weeksboro–Lunksoos Lake, and Munsungun Cambrian–Ordovician inliers in Maine is not consistent with tectonic models involving northwestward migration of arc volcanism. This >150 km span cannot be explained by a single east-facing subduction zone, suggesting more than one subduction zone/arc complex in the region.

A mantle-source solution to the enigma of bimodal arc volcanism

Silicate melts in arc environments are dominated by mafic (low-silica) and silicic (high-silica) compositions, often generating a characteristic bimodal pattern. We investigate the whole arc crust and show that the plutonic lower crust shares the bimodal pattern of melts from volcanoes. This key observation reveals that, contrary to some explanations of bimodal volcanism, variation in mantle source and mantle processes must fundamentally control bimodalism. We also recognise bimodalism in Th/La composition of the whole arc crust and suggest a new working hypothesis: bimodalism originates by melting of distinct sub-arc mantle sources, one dominated by relatively dry peridotite and the other by hydrous pyroxenite. The two groups of primary melts fractionate along distinct liquid lines of descent that lead to relatively dry mafic melts (Th/La~0.1) versus hydrous silicic melts (Th/La>0.2) by 65–80% fractional crystallisation. Common crustal processes such as crystal fractionation, assimilation, reactive flow and/or magma mixing may also lead to differentiation of both groups.