Gaëlle Prouteau

Evidence for mantle metasomatism by hydrous silicic melts derived from subducted oceanic crust.

The low concentrations of niobium, tantalum and titanium observed in island-arc basalts are thought to result from modification of the sub-arc mantle by a metasomatic agent, deficient in these elements, that originates from within the subducted oceanic crust. Whether this agent is an hydrous fluid or a silica-rich melt has been discussed using mainly a trace-element approach and related to variable thermal regimes of subduction zones. Melting of basalt in the absence of fluid both requires high temperatures and yields melt compositions unlike those found in most modern or Mesozoic island arcs. Thus, metasomatism by fluids has been thought to be the most common situation. Here, however, we show that the melting of basalt under both H2O-added and low-temperature conditions can yield extremely alkali-rich silicic liquids, the alkali content of which increases with pressure. These liquids are deficient in titanium and in the elements niobium and tantalum and are virtually identical to glasses preserved in mantle xenoliths found in subduction zones and to veins found in exhumed metamorphic terranes of fossil convergent zones. We also found that the interaction between such liquids and mantle olivine produces modal mineralogies that are identical to those observed in metasomatized Alpine-type peridotites. We therefore suggest that mantle metasomatism by slab-derived melt is a more common process than previously thought.

Fluid-present melting of ocean crust in subduction zones

Phase equilibria were investigated in the pressure range 400–2000 MPa on the Pinatubo dacite, a representative example of natural Cenozoic adakites (slab melts), to constrain the temperature versus wt% H 2 O conditions of magma genesis. The experimental results suggest that water contents of the melt were at least 10 wt% and temperatures were ∼900 °C during magma ascent from the slab. Natural aluminous amphiboles present in the dacite crystallized at pressures of ∼1000 MPa, fixing an upper temperature limit of 950 °C for the slab melts at the crust-mantle boundary. Amphiboles experimentally crystallized at 1000 MPa are identical to those found in melt inclusions in metasomatized mantle xenoliths. These natural inclusions represent samples of slab melts and contain glasses with a dacitic bulk composition, which requires an H 2 O content in the melt of >10 wt% at 1000 MPa and 900 °C. For conditions of magma generation of 800–900 °C at 2000–2300 MPa, melt H 2 O contents close to 15 wt% are needed to generate melts with dacitic bulk composition. Overall, these constraints are inconsistent with slab melts being produced by dehydration melting of amphibolite, a mechanism unable to produce dacite melt compositions that are both hydrous and cool. Slab melting in modern subduction zones must occur under fluid-present conditions at temperatures below 900 °C, in agreement with the thermal regime of subducted oceanic crust as deduced from numerical simulation of heat transfer using the present-day mantle geotherm. At such low temperatures, Ti-bearing phases are stable during melting; thus any metasomatizing slab melt will be strongly depleted in Nb, Ta, and Ti, a major characteristic of arc magmas produced in the mantle wedge.

Continental exhumation triggered by partial melting at ultrahigh pressure

Partial melting textures, observed in most continental crust buried in ultrahigh-pressure (UHP) conditions, have mostly been related to their retrograde evolution during exhumation in collisional orogens. Analysis of leucosomes from the Western Gneiss Region (WGR, Norway) UHP and HP domains in the Caledonides show a wide scatter of their chemistries, from early ones close to trondhjemites restricted to UHP domains, to granites in late occurrences or associated with HP domains. Nearly trondhjemitic compositions compare with hydrous melts produced in felsic systems at high pressure (>2 GPa) and moderate temperature (

Le magmatisme post-collisionnel du Nord-Ouest de Borneo, produit de la fusion d'un fragment de croute oceanique ancre dans le manteau superieur

Magmatic activity linked to syn- or post-collisional zones leads to the emplacement of remarkably heterogeneous rocks: calc-alkaline, high-K calc-alkaline or shoshonitic series variably contaminated by continental crust; anatectic granites and ignimbrites derived from the latter; and finally alkali potassic to ultrapotassic basalts [Harris et al., 1990; Pearce et al., 1984, 1990; Arnaud et al., 1992; Benito et al., 1999]. The main sources of these magmas are either the upper mantle (sub-oceanic or subcontinental) frequently metasomatized by hydrous fluid originating from the subducted slab; or the continental crust, which can act as a contaminant [Benito et al., 1999; Miller et al., 1999] or melt directly [Harris et al., 1990; Zingg et al., 1990]. The purpose of the present paper is to document the role of a third source: the subducted oceanic crust, as evidenced by the occurrence of Miocene adakites in Sarawak (NW Borneo). The studied rocks have been sampled from western Sarawak (fig. 1), and their location is shown on the geological map [Tan, 1982] of figure 2. They mostly occur as stocks, dykes and sills which crosscut the Paleozoic to Miocene sedimentary units. Two kinds of intrusions can be distinguished. High-K calc-alkaline to medium-K calc-alkaline diorites and microdiorites occur in the northern part of the studied area, in Salak Island and Santubong Peninsula. Microtonalites and dacites occur near Kuching and in the southern part of Sarawak (Kuap and Bau areas). Whole-rock K-Ar data (table I) demonstrate that these two associations are of different ages: high-K calc-alkaline diorites were emplaced during the Lower Miocene (22.3 to 23.7 Ma), whereas the microtonalites and dacites are younger by ca. 8 Ma or more (Middle to Upper Miocene, 14.6 to 6.4 Ma). Major and trace element data (table II) show that the Lower Miocene diorites display all the usual characteristics of subduction-related magmas. The Middle to Upper Miocene microtonalites and dacites share some of these characteristics, but in addition they display typical adakitic features: SiO 2 -rich (65.5-70%) and sodic (Na 2 O/K 2 O>2) character (table II and figure 3); lack or rare occurrence of pyroxenes, usually replaced by early-crystallized (near-liquidus) amphiboles (table III); very low Y and HREE contents, consistent with the presence of residual garnet in their source, and leading to characteristically high La/Yb and Sr/Y ratios (fig. 4, 5). Their titanomagnetite-hemoilmenite associations reflect equilibrium features [Bacon and Hirschman, 1988] indicating moderate temperatures (<900 degrees C) and highly oxidizing (NNO+1) crystallization conditions [Ghiorso and Sack, 1991]. The Lower Miocene Sarawak diorites are typically subduction-related from a geochemical point of view. They likely derive from the evolution of island-arc basaltic magmas, which themselves originated from the partial melting of upper mantle peridotites previously metasomatized by hydrous fluids expelled from the subducting oceanic slab [Tatsumi et al., 1986; Tatsumi, 1989]. The origin of the Middle-Upper Miocene adakitic microtonalites and dacites is different. According to previous studies, they likely derive from the partial melting of metabasalts (garnet amphibolites or eclogites) from subducted oceanic crust [Defant and Drummond, 1990; Defant et al., 1991, 1992; Drummond et al., 1996; Maury et al., 1996; Martin, 1993, 1999]. Their position in the hybrid tonalite+peridotite system [Caroll and Wyllie, 1989] shows that they crystallized within the garnet stability field and likely interacted with the upper mantle during their ascent (fig. 7). This feature is not consistent with their genesis through melting of metabasalts accreted at the base of the Borneo continental crust. In addition, the less evolved Sarawak adakites display mineralogical and geochemical features remarkably similar to those of the 1991 Mt Pinatubo dacite, the experimental petrology of which has been extensively studied at low [2 kbar; Scaillet and Evans, 1999; Rutherford and Devine, 1996] to medium pressures [4 to 20 kbar; Prouteau et al., 1999]. Such dacitic magmas are not in equilibrium with garnet at pressures lower than or equal to 20 kbar, which rules out their derivation from metabasalts tectonically or magmatically accreted to the base of the North Borneo continental crust. We propose, instead, that they originated from the partial melting of basalts from a fragment of oceanic lithosphere within the upper mantle. Like the adakites of Central Mindanao, Philippines [Sajona et al., 1994, 1997 and 2000; Maury et al., 1996] and those from Aird Hills, Papua-New Guinea [Smith et al., 1979; Defant and Drummond, 1990] the Sarawak adakites represent potential markers of the occurrence at depth of oceanic crust slivers, which could be much more common in collision zones than previously thought.

Oceanic slab melting and mantle metasomatism

Modern plate tectonic brings down oceanic crust along subduction zones where it either dehydrates or melts. Those hydrous fluids or melts migrate into the overlying mantle wedge trigerring its melting which produces arc magmas and thus additional continental crust. Nowadays, melting seems to be restricted to cases of young (<50 Ma) subducted plates. Slab melts are silicic and strongly sodic (trondhjemitic). They are produced at low temperatures (<1000°C) and under water excess conditions. Their interaction with mantle peridotite produces hydrous metasomatic phases such as amphibole and phlogopite that can be more or less sodium rich. Upon interaction the slab melt becomes less silicic (dacitic to andesitic), and Mg, Ni and Cr richer. Virtually all exposed slab melts display geochemical evidence of ingestion of mantle material. Modern slab melts are thus unlike Archean Trondhjemite–Tonalite–Granodiorite rocks (TTG), which suggests that both types of magmas were generated via different petrogenetic pathways which may imply an Archean tectonic model of crust production different from that of the present-day, subduction-related, one.