J.L. Joron

Isotope and trace element evidence for three component mixing in the genesis of the North Luzon arc lavas (Philippines)

Post-3Ma volcanics from the N Luzon arc exhibit systematic variations in 87Sr/86Sr (0.70327–0.70610), 143Nd/144Nd (0.51302–0.51229) and 208Pb*/206Pb* (0.981–1.035) along the arc over a distance of about 500 km. Sediments from the South China Sea west of the Manila Trench also exhibit striking latitudinal variations in radiogenic isotope ratios, and much of the isotopic range in the volcanics is attributed to variations in the sediment added to the mantle wedge during subduction. However, Pb-Pb isotope plots reveal that prior to subduction, the mantle end-member had high Δ8/4, and to a lesser extent high Δ7/4, similar to that in MORB from the Indian Ocean and the Philippine Sea Plate. Th isotope data on selected Holocene lavas indicate a source with unusually high Th/U ratios (4.5–5.5). Combined trace element and isotope data require that three end-members were implicated in the genesis of the N Luzon lavas: (1) a mantle wedge end-member with a Dupal-type Pb isotope signature, (2) a high LIL/HFS ‘subduction component’ interpreted to be a slab-derived hydrous fluid, and (3) an isotopically enriched end-member which reflects bulk addition (<5%) of subducted S China Sea terrigenous sediment. The 87Sr/86Sr ratios in the volcanics show a restricted range compared with that in the sediments, and this contrasts with 143Nd/144Nd and 208Pb*/206Pb*, both of which have similar ranges in the volcanics and sediments. Such differences imply that whereas the isotope ratios of Nd, Pb and Th are dominated by the component from subducted sediment, those of Sr reflect a larger relative contribution from the slab-derived fluid.

Geochemistry and tectonic setting of the Luzon arc, Philippines

Miocene (10 Ma) to Recent volcanism is associated with eastward subduction along the Manila Trench for some 1,200 km from the Coastal Range in Taiwan south to Min-doro. We suggest calling this the Luzon arc.. There are five distinct segments along this arc: Mindoro, Bataan, Northern Luzon, Babuyan, and Taiwan. The Bataan and Mindoro segments in the south are separated by a northeast-southwest zone of volcanoes. The Northern Luzon segment consists of Pliocene to Pleistocene necks and plugs that run along the entire western section of Northern Luzon. The Babuyan segment (10 Ma to Recent) is made up of a larger number of active volcanoes than are other segments of the Luzon arc. The Taiwan segment is composed entirely of extinct volcanoes in the Coastal Range and on the islands of Lanhsu and Lutao (late Miocene to 1.8 Ma).nnThe nature of volcanism throughout the Luzon are is clearly subduction related with calc-alkaline affinities; high Th/La ratios; and negative Nb, Ta, and Ti anomalies. The earliest phase of volcanism so far recorded (10 Ma), however, is tholeiitic (island of Calayan in the Babuyan segment) with mid-oceanic-ridge basalt-like (MORB-like) tight-rare-earth element and large-ion lithophile element concentrations and low Th/U ratios.nnThere are clear differences in Sr and Nd isotopic ratios among the samples from the Luzon are. Samples from the southern segments (Mindoro through Bataan) fall within the mantle array, with higher Sr and lower Nd isotopic values than those of MORB. Pb isotope data indicate sediment involvement. The isotopic values have been interpreted to result from source contamination by slab-derived fluids (Bataan segment) and by subducted crustal material from collision of the Mindoro-Palawan terrane (Mindoro segment) with the Manila Trench. In contrast, Babuyan segment samples (north) have unusual island-arc Nd and Sr isotopic ratios that fall below the mantle array; they appear to be the result of sediment source contaminants or a unique mantle composition. Incorporation of sediments analyzed from eastern China and Taiwan into a MORB-type mantle can explain the Nd and Sr isotopic ratios of the Babuyan segment.

The geochemistry and tectonic setting of the northern section of the Luzon arc (The Philippines and Taiwan)

Abstract The Luzon arc consists of a 1200 km chain of stratovolcanoes and volcanic necks stretching from Mindoro (13° N) to the Coastal Range of Taiwan (24° N). This study is concerned with three of the five major segments along the arc: the Northern Luzon, Babuyan, and Taiwan segments. The late Tertiary to Quaternary volcanics of these segments are primarily andesitic but range in composition from basalt to rhyolite and are typical arc volcanics: porphyritic plagioclase textures, primarily calc-alkaline with a few tholeiitic volcanic centers, low TiO 2 concentrations, and low high-field strength element (HFSE) to large-ion lithophile element (LILE) ratios. There is a large range in K 2 O and other LILEs from low-K tholeiites to high-K calc-alkaline suites. Calayan island and Mt. Tabungon (volcanic substratum of Mt. Cagua) volcanics in the Babuyan segment and most rocks from the Taiwan Coastal Range are low-K tholeiites (e.g., nearly flat REE patterns, low LREE and LILE concentrations, and low Th/U ratios). The dominant calc-alkaline series ranges from medium-K to high-K rocks. The high-K calc-alkaline rocks are mainly young and are found on Batan and Lutao islands. When the low-K tholeiitic and medium- and high-K calc-alkaline rocks are associated in the same region, the low-K rocks are usually older. There is a general relationship between K 2 O (and other LILE concentrations) and K-Ar radiometric dates. To some extent, 87 Sr 86 Sr ratios also increase with time throughout the arc. There is a correlation between latitude and 87 Sr 86 Sr , and a positive correlation between 87 Sr 86 Sr and [La Sm] CN . An increase in both these geochemical parameters has been associated with an increase in the input of continental crustal material in other arc regions. The lowest 87 Sr 86 Sr ratios found along the entire Luzon arc (the samples from Baguio) are equivalent to those found in samples from mid-Tertiary plutons from Northern Luzon and the Bicol arc. Both groups of rocks are associated with westward subduction along the Philippine Trench where presumably deep oceanic sediments with little or no continental crustal component have been subducted. The absence of continental crust below the arc in the Northern Luzon and Babuyan segments has been suggested by several groups of researchers. This, together with the fact that metasomatized ultramafic nodules (probably of mantle origin) in disequilibrium with their Batan host lavas (but with similar 87 Sr 86 Sr and [La Sm] CN ratios to the host rocks), suggests that upper level assimilation (AFC processes) does not appear to be the major influence on the geochemical signatures. A collision zone between the upper crustal block of Eastern China and Taiwan and the Manila Trench has been recorded in the northern section of the arc (Taiwan). Sediments have also been shown to decrease in thickness along the South China Sea basin nearly parallel to the Manila Trench from north to south. The sediment source region is most probably Taiwan, and perhaps Eastern China. This latitudinal in variation sediment thickness may explain the crustal signature related to both geochemical and age parameters. We suggest that this crustal input has taken place via subduction of sediment rich in a crustal component.

MAGMATIC EVOLUTION OF SULAWESI (INDONESIA) : CONSTRAINTS ON THE CENOZOIC GEODYNAMIC HISTORY OF THE SUNDALAND ACTIVE MARGIN

Abstract Tertiary and Quaternary magmatic rocks from West Sulawesi record the complex history of part of the Sundaland margin where subduction and collision have been and are still active. The present study, based on petrographic data, major- and trace-element chemistry and 40 Kue5f8 40 Ar dating aims to document the age and chemical characteristics of the magmatic formations from West Sulawesi and to determine the corresponding constraints on the geodynamic evolution of the Sundaland border. The West Sulawesi magmatic province includes the South Arm of Sulawesi (Ujung Pandang area), the western part of Central Sulawesi with the Toraja and Palu areas, and finally, the North Arm, extending from Palu to Manado, which includes the Tolitoli and Manado areas. Paleocene magmatic activity seems to be restricted to an episode of calc-alkaline magmatism in the Ujung Pandang area (61-59 Ma). The major Eocene (50-40 Ma) magmatic event is tholeiitic and is documented in all areas except in Ujung Pandang. It led to the emplacement of tholeiitic pillow-lavas and basaltic dykes of back-arc basin (BAB) affinity. These rocks are potential equivalents to the Celebes Sea basaltic basement. From Oligocene to Miocene, magmatic eruptions produced successively island-arc tholeiitic (IAT) and calc-alkaline (CA) rock series. The youngest IAT activity occurred around 18 Ma in the central part (Palu area) and around 14 Ma in the North Arm (Tolitoli area) while CA magmas were emplaced in the North Arm at ca. 18 Ma (Tolitoli and Manado areas). Typical calc-alkaline activity resumed only in the North Arm (Tolitoli and Manado areas) during the Late Miocene (9 Ma) and is still active in the Manado region. In other areas (Palu, Toraja and Ujung Pandang areas) an important and widespread magmatic event occurred between 13 and 10 Ma and emplaced K-rich magmas, either silica-undersaturated alkali-potassic basalts (AK), ultrapotassic basanites (UK) or shoshonites (SH). K-rich activity continued in the south until the Pleistocene (0.77 Ma) with alkali-potassic, ultrapotassic and shoshonitic magmas. In Central Sulawesi (Toraja and Palu areas) the most recent magmatic event occurred between 6.5 and 0.6 Ma. The corresponding products are granitic rocks and widely distributed rhyolitic pyroclastic flow deposits. All these rocks are acidic in character (SiO 2 > 60%), with trace-element and isotopic signatures (Srue5f8Ndue5f8Pb) typical of a strong continental imprint. The most striking tectonic implication of this magmatic evolution is that West Sulawesi can no longer be considered as a typical magmatic arc as previously assumed. With the exception of the Manado area beneath which subduction is still active, calc-alkaline and island-arc tholeiitic lavas and plutonics are volumetrically minor with respect to K-rich magmas. Their occurrence through time is also fairly restricted, mostly to the period between 30 and 15 Ma. Another important feature is the occurrence of island-arc tholeiitic and calc-alkaline magmas crosscutting an older terrane of BAB affinity, the Tinombo Formation (Manado, Tolitoli and Palu areas). As this formation is being regarded as an equivalent to the Celebes Sea floor, the most likely explanation for this feature is the hypothesis of tectonic erosion linked to the NW-dipping subduction beneath the North Arm. The Late Miocene high-K magmatic activity in Central and South Sulawesi reflects the prevalence of a post-collisional tectonic regime following the docking of microcontinents of Australian origin to Central Sulawesi during Neogene times. The incompatible element-enriched character of these high-K rocks might reflect their derivation from a mantle source enriched through metasomatism related to a previous subduction event. Such a model cannot account for the Plio-Pleistocene CAK magmatism of Central Sulawesi, the acidic composition of which does not support a derivation from an ultrabasic source. The trace-element patterns of the CAK rocks are very similar to those of the high-grade metamorphics of Central Sulawesi, suggesting that the latter might represent their possible source. Such an anatectic model implies a collisional to post-collisional tectonic regime limited to Central Sulawesi, while a post-subduction regime prevailed in the south.

Trace element behavior in the alkali basalt-comenditic trachyte series from Mururoa Atoll, French Polynesia

Abstract Numerous drill holes have penetrated close to 1000 m of the volcanic pile of Mururoa atoll. The rocks are a typical example of mildly alkaline intraplate basaltic volcanics ranging from Mg-rich compositions to comenditic trachytes. Evolved basalts and hawaiites are the dominant rock types. Benmoreites and trachytes are relatively uncommon. The available Kue5f8Ar ages indicate a long period of activity between 11.8 and 10.7 Ma. Nevertheless, all the volcanic rocks studied are cogenetic in a broad sense (identical 143 Nd 144 Nd and initial 87 Sr 86 Sr ratios, constancy of ratios of highly incompatible elements). No indications of the occurrence of assimilation or magma mixing have been found. The Mururoa series is thus suitable for the study of fractionation-related processes. Three crystallization/fractionation models have been tested using trace elements, the mineral proportions taken from mass-balance calculations on major elements and individual distribution coefficients determined from trace element data on phenocrysts/host rock pairs. Closed-system fractional crystallization (CSF) is consistent with all of the trace element data. It satisfactorily reproduces the trends observed for compatible and incompatible elements, including the complex behavior of Y and rare-earth elements which are fractionated by kaersutite and apatite in mugearitic and benmoreitic magmas. In situ crystallization and equilibrium crystallization produce trends very close to Rayleighs law model for most incompatible elements. In contrast, in situ and equilibrium crystallization models produce significant underdepletions (compared to CSF) in compatible elements (Sc, Cr, Co, Ni). The corresponding patterns are not consistent with our analytical data in intermediate and evolved rocks. Cooling calculations indicate, however, that the length of volcanic activity in Mururoa ( ≥ 1 Ma) is hardly compatible with closed-system fractional crystallization of a single magma batch. Various models of open-system fractionation have thus been tested, but generally they do not fit the observed trace element patterns. However, one peculiar case of open-system fractionation in a periodically replenished magma chamber is consistent with the data. In this model, batches of mantle-derived parent liquids filling up the chamber at the beginning of each cycle evolve by fractional crystallization. All the corresponding residual liquids either crystallize or erupt at the end of each cycle, before the next replenishment. Thus, the apparently cogenetic Mururoa series is likely to result from fractional crystallization occurring under similar conditions through time, either in several magma chambers or in a single periodically refilled reservoir.

Tertiary and quaternary magmatism in Central Sulawesi: Chronological and petrological constraints

40K-40Ar ages (27 new ones) and geochemical data (major and trace elements) allow the recognition of four main magmatic events in Central Sulawesi (South of Palu and in the Toraja area). The oldes magmatic activity took place during Late Eocene and Oligocene, and led to the emplacement of island arc tholeiites and calc-alkaline magmas as intrusions cutting the Tinombo Formation near Donggala (34.5 My) and the Kambuno granitic body in the Toraja area (29.9 My). The Lamasi Volcanics followed, but their Kue5f8Ar ages, which range from 33 to 15 My, cannot be safely interpreted as magmatic ages. They have a MORB-type affinity, flat to slightly depleted incompatible trace-element patterns and Nb and Ta negative anomalies indicative of their BABB affinity. Their origin and mode of emplacement are still enigmatic. Then an important magmatic event, with a strong shoshonitic affinity, occurred in a short time interval (10.1−11.9 My), leading to the emplacement of intrusions cross-cutting the Mamasa and Kambuno granitic bodies and to the eruption of lava flows (Sekata, Talaya and Sesean Volcanics). This shoshonitic episode is probably equivalent to the post-subduction magmatic associations emplaced in South Sulawesi during the late Miocene and might derive from the melting of mantle material after the mantle was metasomatised during a former episode of subduction. The last magmatic event occurred between 6.5 and 0.6 Ma. The corresponding products are granitic rocks (Kulawi, Kamarora, Palopo), rhyolites (Gimpu, 1.9 My) and widely distributed rhyolitic pyroclastics known as the Barupu tuff (0.6 Ma). All these magmatic rocks are characterised by their K-rich calc-alkaline composition, by the lack of basaltic and intermediate magmas (SiO2 > 60%) and by high enrichments in incompatible elements a radiogenic Sr isotopic signature. This is consistent with the hypothesis of a strong crustal imprint by melting of an underthrusted continental crust in a collisional context.

Melting of lower continental crust in a young post-collision setting; a geochemical study of Plio-Quaternary acidic magmatism from central Sulawesi (Indonesia)

Acidic potassic calc-alkaline (CAK) magmas have been emplaced in the central part of the western arm of Sulawesi from 6.5 to 0.6 Ma, mostly as peraluminous dacites, rhyolites and granites. They overlay or crosscut a high-grade metamorphic basement including lower crustal garnet peridotites and granulites, the latter showing evidences for incipient melting during rapid uplift. Major and trace element data coupled with a Sr, Nd and Pb isotopic study of the CAK magmas and their lower crustal basement rocks demonstrate that they share a number of common features, including radiogenic Sr and Pb and unradiogenic Nd signatures, consistent with those of Australian granulites and Indian Ocean sediments. We propose that the CAK magmas derived from the anatexis of lower crustal rocks of Australian origin (the Banggai-Sula microcontinent) during the phase of uplift which followed their collision with the Sundaland margin (the western arm of Sulawesi) during the Middle Miocene, and possibly the breakoff of the subducted Molucca Sea slab.