Martin F. J. Flower

Magmatism in the South China Basin

Abstract A widespread episode of intraplate volcanism followed the cessation of sea-floor spreading in the South China Basin (≈32-17 Ma BP), affecting large parts of southern China and Indochina, and penetrating oceanic basement and stranded microcontinent fragments. Geochemical data for post-spreading seamount and island lavas define suites of quartz tholeiite, olivine tholeiite, alkali olivine basalt and nephelinite, characterised by OIB-type incompatible-element distributions. High-K alkalic lavas show extreme enrichment in large-ion lithophile and high-field-strength elements relative to N-MORB. 87 Sr 86 Sr and 143 Nd 144 Nd ratios are depleted relative to bulk Earth values and partially overlap with Central Indian Ridge MORB and associated OIB. In contrast, 208 Pb 204 Pb and 207 Pb 204 Pb ratios are variable and surprisingly radiogenic for given MORB-like 206 Pb 204 Pb . The isotopic and trace-element systematics confirm source heterogeneity but appear to be decoupled, implying complex mantle enrichment histories. At least two heterogeneous source components are required: a depleted but “contaminated” Indian Ocean MORB type, and an EM-2 reservoir whose isotopic composition corresponds to continent-derived sediment. Dupal-like Pb isotopic compositions ( Δ7 4 Pb = 2−13 , Δ8 4 Pb = 45−73 ) are shared by intraplate basalts from Hainan Island, the Penghu Islands, northern Taiwan and post-collision arc basalts from the Philippines. It is proposed these reflect endogenous mantle processes related to disaggregation of the south China margin rather than a northward extension of the southern hemisphere Dupal anomaly.

Sr, Nd, and Pb isotopic compositions of Hainan basalts (south China): Implications for a subcontinental lithosphere Dupal source

Sr, Nd, and Pb isotopic ratios were determined for postspreading basalts from Hainan Island, erupted in a quasi-oceanic pull-apart trough at the northern edge of the South China Basin. The basalts comprise quartz tholeiite, olivine tholeiite, alkali basalt, and basinite and show oceanic island basalt (OIB)-type trace element distributions relatively enriched in Nb, Ta, and Th. Despite normal OIB-like Sr and Nd isotopic compositions, Hainan lavas have unexpectedly high {sup 208}Pb/{sup 204}Pb and {sup 207}Pb/{sup 204}Pb ratios, resembling those of Southern Hemisphere Dupal OIB and some alkalic arc basalts. The authors propose that the Hainan source comprises a lower lithospheric region of accreted asthenospheric melt (isotopically resembling Central Indian Ridge-type basalt) overprinted by radiogenic melts of subducted sediment. In the absence of a mantle plume, decompression melting of subcontinental lithospheric mantle may result from lithosphere stretching, as a regional response to the Indo-Eurasian collision.

Arc-trench rollback and forearc accretion: 2. A model template for ophiolites in Albania, Cyprus, and Oman

Abstract Ophiolite assemblages record structural, magmatic, and metamorphic processes that preceded their entrapment in orogenic belts by continental plate collisions. Ophiolite genetic models appealing to ‘oceanic’ or ‘suprasubduction’ provenance are still unable to reconcile several basic problems, including: (1) the association of boninites with oceanic ridge-type structural settings; (2) the diachronous ‘patch-like’ distribution of ophiolites in orogenic belts; (3) disparate ages between and within their mantle and crustal sections; (4) the lack of evidence for ‘obduction’ at modern passive margins. In contrast, the proposal that ophiolite genesis is exclusive to intra-oceanic forearc settings is compelling, given their uniquely shared structural, lithological, and stratigraphic attributes. Forearcs are interpreted to record discrete stages of subduction ‘rollback’ cycles, examples of which begin with subduction nucleation and the formation of boninitic ‘proto-arcs’, followed by arc splitting and concomitant retreat of the evolving arc-forearc complex. Forearc assemblages are likely to resist subduction to become entrapped in orogens, in contrast to denser, recently formed back-arc basin lithosphere, which is reconsumed by subduction following collision of the retreating forearc. As a model for Neo-Tethyan ophiolite genesis, this is predicated on the notion that rollback cycles are driven by ductile asthenosphere mobilized prior to and during collisions of Gondwana fragments with accreting Eurasia. It is also consistent with the apparent correlation of ophiolite ages with collisional events and their conjugate plate kinematic adjustments. Here, we use the slab rollback model as a template for interpreting the structural, magmatic, and metamorphic characteristics of well-studied Tethyan ophiolites, in Albania (Mirdita), Cyprus (Troodos), and Oman (Semail).

Magmatism in the South China Basin: 2. Post-spreading Quaternary basalts from Hainan Island, south China

Cenozoic magmatism on northern Hainan Island was part of a regional episode post-dating opening of the South China Basin (SCB). Tholeiite lava flows emanated from WSW-ENE-trending extensional fissures and are interlayered with mid-Miocene to Quaternary sediments, forming a 200–1000-m-thick sequence. Subsidiary alkali basalts were erupted from central volcanoes aligned on NW-SE-trending strike-slip faults, and in some cases contain ultramafic xenoliths, and megacrysts of augite, sapphire (corundum), zircon and anorthoclase. Cumulative thickness of the lavas rarely exceeds 200 m although sills of unknown age intrude early Oligocene to Cretaceous sediments at depths up to 3000 m. Lava compositions include quartz tholeiite, olivine tholeiite, alkali olivine basalt and basanite, and are subdivided into chemical types on the basis of temporal-spatial association. With time the basalts change from relatively thin intercalated quartz tholeiite and alkali olivine basalt flows to more massive olivine tholeiites. In common with other SCB eruptives, Hainan basalts resemble Dupal-type oceanic island basalt and reflect mixing of EM2 and N-MORB mantle reservoirs. Decoupled systematics of Sm/Nd and 143Nd144Nd are at least partly due to differentiation during partial melting, possibly by decompression melting of plagioclase/spinel- and spinel-lherzolite. It is proposed that EM2 derives from sub-continental lithosphere or asthenosphere reservoirs, enriched by subduction prior to crustal extension.

Major, trace element, and isotopic compositions of Vietnamese basalts: Interaction of hydrous EM1-rich asthenosphere with thinned Eurasian lithosphere

Abstract Intraplate magmatism affected much of Indochina following the mid-Miocene cessation of South China Sea opening. Thick basalt plateaus formed on accreted terrains of varying age as extensional fractures were reactivated following the Indo-Eurasian collision. The basalts are part of a diffuse igneous province affecting much of eastern and southeastern Asia and western Pacific marginal basins. Most Indochina basalt centers comprise two eruptive episodes, an early (lower) series of high-Si0 2, low-FeO * quartz and olivine tholeiites, tapping a relatively-refractory, lithospheric mantle-type source, and a later (upper) series of low-SiO 2, high-FeO * olivine tholeiites, alkali basalts, and basanites, tapping a fertile, asthenospheric source. This pattern is observed elsewhere in the region (e.g., Hainan Island) and resembles several continental flood basalt provinces. While some crustal contamination is suggested, incompatible trace element and strontium, neodymium, and lead isotopic compositions reflect secular changes from the inferred lithospheric to asthenospheric reservoirs. Lower Series basalts reflect hybrids of 206Pb/ 204Pb-rich EM2 and N-MORB reservoirs, with high K 2O/P 2O 5 and low Rb/Sr and Ba/Nb ratios, consistent with the involvement of lithospheric mantle. In contrast, Upper Series basalts show lower K 2O/P 2O 5 and higher Rb/Sr and Ba/Nb ratios and reflect hybrids of 206Pb/ 204Pb-poor EM1 and N-MORB sources. These resemble anomalous (A) -MORB compositions that are typical of eastern/southeastern Asian and western Pacific marginal basin asthenosphere. Despite its resemblance to Indian Ocean (I-) MORB, A-MORB “plum-pudding” asthenosphere may be explained in terms of an endogenous Asian model whereby EM1-rich subcratonic lithosphere was entrained by asthenosphere extruded by the Indo-Eurasian collision. This model is consistent with the restriction of diffuse regional magmatism to the late Cenozoic (i.e., following tectonic extrusion), evidence for shallow, thermally-anomalous mantle, and absence of A-MORB signatures from the pre-extrusion continental mantle.

Petrogenesis of a tholeiite-boninite sequence from Ayios Mamas, Troodos ophiolite: evidence for splitting of a volcanic arc?

Lavas and intrusives from Ayios Mamas, at the western end of the Limassol Forest Complex, south of the Arakapas Fault Belt, Troodos, comprise three groups: (A) a basal sequence of pillow lavas (intruded by dikes and sills) with the phenocryst assemblage PLAG+CPX+MT±OL; (B) a middle sequence of pillows and massive flows [CPX+PLAG±OL]; and (C) an upper sequence mostly of pillows and breccia [OL+OPX+CPX+SP]. The lower group is tholeiitic, and the upper two groups boninitic. Group C whole-rock and glass Mg-numbers exceed 72 and range up to 78, with corresponding olivine compositions of Fo92. They are strongly depleted in high-field strength elements, and on the basis of Cr-Y variation and major element mass balances, are interpreted to reflect partial melting of a basalt-depleted (CPX-) harzburgite source. Group A tholeiites, in contrast, are interpreted to derive from a more fertile lherzolite source. Magmaphile and compatible trace element variation is best modelled after an open magma system, involving periodic replenishment, tapping, and fractionation, although neither tholeiites nor boninites represent “steady-state” compositions. The boninites belong to a CaO-rich, SiO2-poor variant, resembling those from Guam, but contrasting with those from Bonin and Cape Vogel. MORB-normalized enrichment of low-field strength elements K, Sr, Ba, and Rb in all groups indicates subduction-derived source contamination. It is proposed that the Ayios Mamas section reflects splitting of a volcanic arc, prior to back-arc spreading in an environment analogous to that of the present-day Andaman Sea.

The genesis and significance of N-MORB sub-types

A global compositional dichotomy for N-MORB magma (N1/N2) is recognized on the basis of Na2O, TiO2, CaO, and Al2O3 contents, and their respective ratios. We have characterized the two magma sub-types by means of their trace element patterns, and attempted to explain the differences in major and trace element contents in terms of a partial melting model, using data from DSDP/IPOD Leg 82. Mass balance calculations for N-MORB glass and rock compositions indicate that differences between N1-and N2-MORB are consistent with simple differences (5%–10%) in the degree of partial melting of a plagioclase-(±spinel) lherzolite, at pressures <10 kbar, rather than their respective derivation from plagioclase- and spinel-lher-zolite sources. Based on published and calculated partition coefficients, and calculated source magmaphile trace element compositions, the calculations indicate that the overall range of N-MORB compositions may be derived by between approximately 8% and 20% partial melting of a fertile lherzolite source. Fluid dynamic and melt kinematic considerations will probably necessitate refinement of the model, but should also take account of its qualitative precepts.

The Nd-, Sr- and Pb-isotopic character of lavas from Taal, Laguna de Bay and Arayat volcanoes, southwestern Luzon, Philippines: Implications for arc magma petrogenesis

Abstract Following the amalgamation of a collage of pre-Neogene terranes largely by strike-slip and convergence mechanisms to form the Philippine islands, volcanic chains, related to oppositely dipping subduction zones, developed along the eastern and western margins of the archipelago. There is ample field evidence that this volcanic activity, predominantly calc-alkaline in chemical character, had commenced by the Oligocene. Volcanoes resulting from subduction along the Manila-Negros trench in the west (e.g. Taal, Laguna de Bay and Arayat) form a high-angle linear array, trending away from the MORE field on Pb-isotopic covariation diagrams; have the highest Sr- and lowest Nd-isotopic compositions, of the two chains (but nevertheless plotting above bulk earth on the 87 Sr/ 86 Sr versus 143 Nd/ 144 Nd covariation diagram); and exhibit Sm/Nd and Rb/Sr values that are lower and higher, respectively, than the estimated values for bulk earth. While the Sm/Nd and Rb/Sr characteristics are common to both chains, volcanoes associated with the Philippine-East Luzon trench have Pb-isotopic compositions that fall in the Indian Ocean MORB field and that require time-integrated evolution in a high Th/U environment. They also have higher Nd- and lower Sr-isotopic ratios. The source materials of Philippine volcanoes, therefore, have undergone varied recent enrichments in LILE, as indicated by the decoupling of isotopic and elemental ratios. These enrichments, particularly for the western volcanoes, cannot be entirely due to small degrees of partial melting in the mantle wedge, considering that they were accompanied by elevations in radiogenic Pb. Elevated Pb ratios are best explained by the introduction of subducted, continentally derived sediments. The sedimentary component in the western volcanoes is probably the South China Sea sediments derived largely from Eurasia. That this component is not available in the Philippine-East Luzon trench is reflected by the fact that the eastern volcanoes have higher Nd- and lower Sr-isotopic ratios as well as less radiogenic common Pb.

Genesis of the Kinabalu (Sabah) granitoid at a subduction-collision junction

The Kinabalu batholith is a late Neogene granitoid in northwestern Sabah (East Malaysia) apparently marking the locus at which subducted South China Basin lithosphere interacted with roots of the northern Sabah collision suture. The exposed batholith comprises a relatively small core of biotite quartz monzodiorite (BQM) grading out to dominant hornblende quartz monzonite (HQM). Both lithologies contain mafic igneous and metasedimentary inclusions and are cut by late-stage aplite dikes. Major element data indicate the BQM (K2O/Na2O ratios ranging 0.72–1.03) represents a low-K type while HQM (K2O/Na2O ratios ranging 1.35–5.58) is a distinct high-K type. Magmaphile element distributions support this distinction, HQM showing higher K, Rb, LREE and lower Ta and Nb contents than BQM, indicating the more extensive interaction of HQM with sial. Least-squares mass balance models suggest that HQM evolved through the combined effects of fractional crystallization and crustal assimilation while BQM was dominated by fractional crystallization. However, similar plagioclase zoning patterns in both lithologies suggest they are comagmatic rather than generated by melting of separate sources. It is concluded that low-K type melts, formed by subduction-induced remelting of underplated lower crust, underwent high pressure sialic contamination with development of high-K character. These provided access to later-formed low-K melts which were less contaminated and consolidated to form the pluton core. The unusual zonation from inner low-K to outer high-K type compositions may indicate that the cessation of subduction prevented upward migration of the melting anomaly and thermal maturation of the pluton.

Subduction-modified subcontinental mantle in South China: Trace element and isotope evidence in basalts from Hainan Island

Cenozoic lavas from Hainan Island, South China, comprise quartz tholeiite, olivine tholeiite, alkali basalt, and basanite and form a continuous, tholeiite-dominated, compositional spectrum. Highly incompatible elements and their relationships with isotopes in these lavas are shown to be useful in evaluating mantle-source composition, whereas modeling suggests that ratios of elements with bulk partition coefficients significantly larger than those of Nb and Ta may be sensitive to partial melting. Th/Ta and La/Nb ratios of alkali basalts are lower than those of tholeiites, and they are all lower than those of the primitive mantle. These ratios correlate positively with207Pb/204Pb and87Sr/86Sr ratios. Such relationships can be explained by mixing of depleted and enriched source components. A depleted component is indicated by alkali basalt compositions and is similar to some depleted OIB (PREMA). The enriched component, similar to sediment compositions, is indicated by tholeiites with high LILE/HFSE,207Pb/204Pb, and87Sr/86Sr ratios.In general, basalts from Hainan and the South China Basin (SCB) share common geochemical characters, e.g. high Rb/Sr, Th/Ta,207Pb/206Pb, and low Ba/Th ratios. Such a geochemical trend is comparable to that of EMII-type OIB and best explained as the result of subduction. Occurrence of these characteristics in both continental Hainan basalts and SCB seamount basalts indicates the presence of a South China geochemical domain that exists in the mantle region below the lithosphere.