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Journal ArticleDOI

Adakites without a slab: Remelting of hydrous basalt in the crust and shallow mantle of Borneo to produce the Miocene Sintang Suite and Bau Suite magmatism of West Sarawak

01 Nov 2019-Lithos (Elsevier)-Vol. 344, pp 100-121

AbstractWe present new geochronological and geochemical data for Neogene magmatism from West Sarawak. Zircon U-Pb geochronology divides Neogene magmatic rocks of West Sarawak into a Lower Miocene West Sarawak Sintang Suite with ages of c. 19 to 21 Ma, and a Middle Miocene Bau Suite with ages of c. 12 to 14 Ma. Magmatism occurred in multiple short-lived pulses from approximately 24 Ma and was coeval with magmatic activity in NW Kalimantan and East Kalimantan. The majority of, but not all, Bau Suite samples display adakitic chemistry, while the West Sarawak Sintang Suite is predominantly non-adakitic. There was no active subduction zone or subducted slab associated with this adakitic magmatism. Instead, the geochemical diversity is consistent with the Bau and West Sarawak Sintang suites representing mixtures of mafic, mantle-derived magma with felsic magma derived from remelting of hydrous, mafic rock that had been emplaced into the lithosphere of Borneo as arc basalt tens or hundreds of millions of years previously. This origin is most evident in the main Sintang Suite of central Borneo (Kalimantan) which has preserved less contaminated examples of the mafic endmember. This endmember resembles basaltic rocks from several locations across Borneo suggesting that intraplate, mantle-derived magmatism was responsible for remelting older, hydrated basaltic rocks in the crust.

Topics: Adakite (57%), Mafic (55%), Felsic (55%), Magmatism (54%), Basalt (51%)

Summary (3 min read)

1. Introduction

  • Subduction zones have been major sites of crustal processing since at least the Neoproterozoic.
  • Thus, several different processes – some involving slab melting, some not – have been proposed to explain the generation of the adakitic chemical signature.
  • The authors explore the temporal, petrological, and geochemical development of a suite of Neogene magmatic rocks from Borneo that includes adakitic rocks.
  • These were generated in a setting that had lacked subduction during, at least, the preceding 50 million years, and where there is no evidence of substantial crustal thickening.

2. Regional background

  • Geochemically similar rocks from Kalimantan and West Sarawak (Kirk, 1968; Williams and Harahap, 1987) are predominantly of Late Oligocene to Early Miocene age, and have been referred to as the Sintang Intrusives, the Sintang Intrusive Suite or the Sintang Suite (e.g. Doutch, 1992; Moss et al., 1998; Hutchison, 2005, 2010).
  • Compositions are predominantly dacitic, granodioritic, or subordinately dioritic to granitic, with Itype character (Williams and Harahap, 1987).
  • Whole-rock, biotite, and hornblende K-Ar dating of 12 samples collected near Sintang in NW Kalimantan (Williams and Harahap, 1987) yielded two distinct age groups: an older group of 30.4 to 23 Ma in the Melawi Basin near Sintang (type locality), and a younger group of 17.9 to 16.4 Ma in the Ketungau Basin.

3. Methodology

  • 1. Sampling Fresh rocks or rocks with minimal alteration were sampled (TB samples) from outcrops or nearby float in West Sarawak (Fig. 3).
  • An example of pellet reproducibility, and comparison between XRF and isotope dilution data, are given in the web link referred to above.
  • Grains were mounted in epoxy resin blocks and AC C EP TE D M AN U SC R IP T polished to expose mid-grain sections.
  • Tera-Wasserburg plots were used to identify individual peaks or visually assess outliers (e.g. lead loss, inheritance and common lead) within the population which were then excluded from the weighted mean age calculation.

4. Petrography

  • Intrusive rocks Micro-tonalites/granodiorites (TB33, TB148a, STB36c, and STB61b) dominate the intrusive West Sarawak Sintang Suite.
  • Plagioclase and alkali feldspar form large idiomorphic to hypidiomorphic phenocrysts.
  • Biotite is often replaced by sericite, chlorite and titanite.
  • Idiomorphic volcanic quartz commonly has a bipyramidal shape, embayments and inclusions of sericite, biotite and plagioclase.
  • Plagioclase is zoned and forms idiomorphic to subidiomorphic crystals (Fig. 4k).

5. Geochemistry

  • Intrusive rocks of the West Sarawak Sintang Suite are predominantly felsic with a range of SiO2 contents from 56 to 70 wt. %, classified as granodiorite, monzodiorite and gabbro-diorite (Supplementary Fig. 1).
  • Major element variations in the West Sarawak Sintang intrusive rocks are also coherent with those of Sintang intrusive rocks from Kalimantan (Fig. 5a).
  • This means that their trace element ratios broadly resemble volcanic AC C EP TE D M AN U SC R IP T arc or post-collision rocks (Supplementary Fig. 1).
  • This makes the Bau Suite more similar to Kalimantan’s Central Sintang group than the Northern or Southern groups (Fig. 5).

6. U-(Th)-Pb zircon geochronology

  • Intrusive rocks Sample TB63b TB63b is a granodioritic sill intruding the Kayan Sandstone at Tanjung Santubong.
  • Simple internal zoning is evident in most grains.
  • Concentric, patchy and sector zoning are rare.
  • Two outliers of Miocene age were excluded from the weighted mean age calculation because of lead-loss, resulting in a unimodal population (Fig. 9a) of 81 Miocene ages (98% of total Miocene ages) that cluster between 19 and 23 Ma with a weighted mean age of 21.1 ± 0.2 Ma (MSWD = 3.5).

Sample TB58

  • TB58 is a stock that intrudes sediments of the Silantek Formation sampled from a granodiorite boulder in a small gully from Bukit Kelambi .
  • Zircons are angular, with a euhedral to subhedral or anhedral shape.
  • Simple internal zoning is evident in most grains.
  • A single Miocene age was excluded because of high common lead.
  • The population is predominantly Early Miocene (43 ages) with 7 inherited zircons of Mesozoic to Permian age, ranging from 114 to 267 Ma. 7 outliers of Miocene ages (grey in Fig. 8b) have either lead-loss or inheritance, and were excluded from the weighted mean age calculation.

Sample TB33

  • Simple internal zoning is evident in most grains.
  • Two ages were excluded because of partial ablation of the resin mount.
  • Six Miocene outliers, including a small population of inherited Miocene zircons at around 24 Ma and a number of zircons affected by lead-loss, were identified and excluded from the weighted mean age calculation.

Sample TB209a

  • Sample TB209a was sampled from a rhyolite boulder field near Bukit Buwaya.
  • Inherited zircons are subrounded to anhedral, usually with sector or oscillatory zoning.
  • Two inherited ages were excluded for failing the 10% discordance criteria and two Miocene ages were also excluded because of abundant common lead.
  • Of the valid ages 19 are Miocene, with three Miocene outliers either affected by lead-loss or inheritance (marked in grey) and excluded from the weighted mean age calculation, leaving a unimodal population which includes 16 of 19 (84% of all).

Sample TB61

  • Sample TB61 is a micro-granodiorite collected from the Bukit Stapok quarry in Batu Kawa near Kuching that contains euhedral, elongate zircons with simple or oscillatory zoning.
  • Larger zircons are anhedral, can be easily distinguished from elongate varieties, and are inherited.
  • The sample has one concordant inherited Proterozoic age around 850 Ma.

7. Discussion

  • West Sarawak Sintang Suite The authors U-Pb dating of zircons in volcanic and intrusive rocks from West Sarawak yielded a restricted range of ages (Fig. 10a), suggesting relatively short-lived Miocene magmatic episodes.
  • A single, slightly older, zircon age of c. 19 Ma in TB61, indistinguishable from the age of West Sarawak Sintang rocks (Fig. 10a) suggests Early Miocene magmatism in the Bau area.
  • Thus, melting of hydrated basalt in the mid- to deep-crust below Borneo could produce much of the major element variation character of non-adakitic rocks of West Sarawak, and in the Northern and Southern groups of the Kalimantan Sintang Suite.
  • The presence of such rocks among the Sintang Suite has two important implications.
  • Instead, the authors have identified that emplacement of these suites was accompanied by contemporaneous, mafic, mantle -derived magmatism.

Conclusions

  • Inherited zircons in the West Sarawak Sintang Suite suggest magmatism was active by c. 24 Ma. 2. The Neogene magmatism was not related to active subduction.
  • Geochemistry shows an adakite character for the Bau Suite while the Sintang Suite samples plot predominantly outside the adakite field.
  • The geochemical character of both suites is consistent with remelting of hydrous mafic rocks in the lithosphere of Borneo that were emplaced as arc basalt tens or hundreds of millions of years previously.
  • The mechanisms that generated this magmatism could have provided the heat to re-melt the crust, which yielded the intermediate and evolved intrusive rocks of the Sintang and Bau suites.
  • These may have been relicts from the extension which formed the Melawi and Ketungau basins and/or products of contemporaneous extension/transtension.

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Citation for published item:
Breitfeld, H. Tim and Macpherson, Colin and Hall, Robert and Thirlwall, Matthew and Ottley, Chris J. and
Hennig-Breitfeld, Juliane (2019) 'Adakites without a slab : remelting of hydrous basalt in the crust and
shallow mantle of Borneo to produce the Miocene Sintang Suite and Bau Suite magmatism of West Sarawak.',
Lithos., 344-345 . pp. 100-121.
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Accepted Manuscript
Adakites without a slab: Remelting of hydrous basalt in the crust
and shallow mantle of Borneo to produce the Miocene Sintang
Suite and Bau Suite magmatism of West Sarawak
H. Tim Breitfeld, Colin Macpherson, Robert Hall, Matthew
Thirlwall, Chris J. Ottley, Juliane Hennig-Breitfeld
PII: S0024-4937(19)30256-7
DOI: https://doi.org/10.1016/j.lithos.2019.06.016
Reference: LITHOS 5114
To appear in: LITHOS
Received date: 29 January 2019
Accepted date: 11 June 2019" role="suppressed
Please cite this article as: H.T. Breitfeld, C. Macpherson, R. Hall, et al., Adakites without
a slab: Remelting of hydrous basalt in the crust and shallow mantle of Borneo to produce
the Miocene Sintang Suite and Bau Suite magmatism of West Sarawak, LITHOS,
https://doi.org/10.1016/j.lithos.2019.06.016
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ACCEPTED MANUSCRIPT
Adakites without a slab: Remelting of hydrous basalt in the crust and shallow mantle of
Borneo to produce the Miocene Sintang Suite and Bau Suite magmatism of West Sarawak
H. Tim Breitfeld
1,*
tim.breitfeld@rhul.ac.uk, Colin Macpherson
2
, Robert Hall
1
, Matthew Thirlwall
3
,
Chris J. Ottley
2
, Juliane Hennig-Breitfeld
1
1
SE Asia Research Group, Department of Earth Sciences, Royal Holloway University of London,
Egham, Surrey, TW20 0EX, United Kingdom
2
Department of Earth Sciences, University of Durham, Durham, DH1 3LE, United Kingdom
3
Department of Earth Sciences, Royal Holloway University of London, Egham, Surrey, TW20 0EX,
United Kingdom
*
Corresponding author.
Abstract
We present new geochronological and geochemical data for Neogene magmatism from West
Sarawak. Zircon U-Pb geochronology divides Neogene magmatic rocks of West Sarawak into a Lower
Miocene West Sarawak Sintang Suite with ages of c. 19 to 21 Ma, and a Middle Miocene Bau Suite
with ages of c. 12 to 14 Ma. Magmatism occurred in multiple short-lived pulses from approximately
24 Ma and was coeval with magmatic activity in NW Kalimantan and East Kalimantan. The majority
of, but not all, Bau Suite samples display adakitic chemistry, while the West Sarawak Sintang Suite is
predominantly non-adakitic. There was no active subduction zone or subducted slab associated with
this adakitic magmatism. Instead, the geochemical diversity is consistent with the Bau and West
Sarawak Sintang suites representing mixtures of mafic, mantle-derived magma with felsic magma
derived from remelting of hydrous, mafic rock that had been emplaced into the lithosphere of
Borneo as arc basalt tens or hundreds of millions of years previously. This origin is most evident in
the main Sintang Suite of central Borneo (Kalimantan) which has preserved less contaminated
examples of the mafic endmember. This endmember resembles basaltic rocks from several locations
ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
across Borneo suggesting that intraplate, mantle-derived magmatism was responsible for remelting
older, hydrated basaltic rocks in the crust.
Keywords: adakites; Sintang Suite; Bau Suite; intraplate magmatism; zircon U-Pb geochronology;
Sarawak; Borneo
1. Introduction
Subduction zones have been major sites of crustal processing since at least the Neoproterozoic.
While there is debate about early Precambrian geodynamics, including the role and importance of
subduction (Stern, 2005; van Hunen and Moyen, 2012), modern subduction zones have produced , so
called, adakitic magmatic rocks that resemble the tonalite trondhjemite granodiorite (TTG) suites
which are common constituents of felsic Archean terranes (Campbell and Taylor, 1983; Kelemen,
1995; Drummond et al., 1996). Therefore, understanding the genesis of adakitic rocks is an
important step in understanding the development of the Earth’s continental crust.
Adakites were initially interpreted as melts derived from young subducted oceanic crust (Defant and
Drummond et al., 1990), but there have since been many studies that have found adakitic rocks
either in subduction zones lacking subducted young oceanic lithosphere (e.g. Sajona et al., 1993;
Castillo et al., 1999; Macpherson et al., 2006), or formed by melting of basaltic rock in the highly
thickened crust of collision zones (Chung et al., 2003; Hou et al., 2004; Guo et al., 2007). Thus,
several different processes some involving slab melting, some not have been proposed to explain
the generation of the adakitic chemical signature. Each of these has implications for the geodynamic
settings in which adakites are found and potentially for processes that might have been common
during the Archean.
In this paper, we explore the temporal, petrological, and geochemical development of a suite of
Neogene magmatic rocks from Borneo that includes adakitic rocks. These were generated in a
setting that had lacked subduction during, at least, the preceding 50 million years, and where there
is no evidence of substantial crustal thickening. We show that these adakites were
ACCEPTED MANUSCRIPT

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contemporaneous with (i) non-adakitic granodiorites which were derived from similar sources to the
adakites, and (ii) mantle-derived magmas resembling ocean island basalts. This indicates that the
hydrated basaltic source of the adakites was present in the Borneo lithosphere, and implies
subduction before the Oligo-Miocene, but there was no active subduction zone or subducted slab
associated with the adakitic melts, which probably resulted from intraplate processes.
2. Regional background
The Kuching Zone in Borneo of Haile (1974), extending from the Lupar Line in the north to the
Schwaner Mountains in the south (Fig. 1 and 2), includes Palaeozoic to Cenozoic metamorphic,
sedimentary and igneous rocks (e.g. Liechti et al., 1960; Hutchison, 2005; Breitfeld et al., 2017,
2018). The upper Cenozoic in the Kuching Zone is characterised by widespread, small igneous
intrusions which form the focus of this study (Fig. 2). Geochemically similar rocks from Kalimantan
and West Sarawak (Kirk, 1968; Williams and Harahap, 1987) are predominantly of Late Oligocene to
Early Miocene age, and have been referred to as the Sintang Intrusives, the Sintang Intrusive Suite or
the Sintang Suite (e.g. Doutch, 1992; Moss et al., 1998; Hutchison, 2005, 2010). We follow Hutchison
in preferring the term Sintang Suite because not all of the igneous rocks are intrusive.
2.1. Pre-Oligocene magmatism in Borneo
The extensive Schwaner Mountains granitic batholith, which lies immediately south of the area of
Sintang Suite magmatism (Fig. 2) formed during Cretaceous subduction that ceased at around 90 to
80 Ma (Pieters and Sanyoto, 1993; Hutchison, 1996; Moss, 1998; Hall, 2012; Davies et al., 2014;
Breitfeld et al., 2017; Hennig et al., 2017). Subsequent minor magmatic episodes produced the upper
Cretaceous Pueh and Gading batholiths of West Sarawak (Kirk, 1968; Hennig et al., 2017; Fig. 3), the
Eocene Muller Volcanics, Nyaan Volcanics, Piyabung Volcanics and Serantak Volcanics in NW and
central Kalimantan (Pieters et al., 1987; Bladon et al., 1989; Fig. 2), and the Eocene Piring stock in
North Sarawak (Hennig-Breitfeld et al., 2019; Fig. 3) .
ACCEPTED MANUSCRIPT

Citations
More filters

01 Dec 2008
Abstract: The Philippine Trench marks a nascent plate margin where subduction initiation is propagating from north to south. Magma compositions in the East Philippine Arc record thinning of arc lithosphere as it is eroded from below. Lithosphere is thicker beneath the younger, southern part of the arc, causing basaltic magma to stall and fractionate garnet at high pressure. In the mature, northern section, basaltic magma differentiates at shallower levels, at pressures where garnet is not stable. Local variations in lithosphere thickness suggest that thinning is rapid and may be piecemeal. Fluctuations in arc lithosphere thickness throughout the history of this margin appear to control spatial and temporal variations in magma fluxes into the arc crust. Varying fractionation depths of hydrous basalt may help to explain the andesitic composition of bulk continental crust.

50 citations


Journal ArticleDOI
Abstract: The Schwaner Mountains in southwestern Borneo form a large igneous province with a complex magmatic history and poorly known tectonic history. Previously it was known that Cretaceous granitoids intruded metamorphic rocks of the Pinoh Metamorphic Group assumed to be of Paleozoic age. Jurassic granitoids had been reported from the southern Schwaner Mountains. Most ages were based on K-Ar dating. We present new geochemistry, zircon U-Pb and 40Ar/39Ar age data from igneous and metamorphic rocks from the Schwaner Mountains to investigate their tectono-magmatic histories. We subdivide the Schwaner Mountains into three different zones which record rifting, subduction-related and post-collisional magmatism. The Northwest Schwaner Zone (NWSZ) is part of the West Borneo Block which in the Triassic was within the Sundaland margin. It records Triassic to Jurassic magmatism during early Paleo-Pacific subduction. In contrast, the North Schwaner Zone (NSZ) and South Schwaner Zone (SSZ) are part of the SW Borneo (Banda) Block that separated from NW Australia in the Jurassic. Jurassic granitoids in the SSZ are within-plate (A-type) granites interpreted to have formed during rifting. The SW Borneo (Banda) Block collided with eastern Sundaland at c. 135 Ma. Following this, large I-type granitoid plutons and arc volcanics formed in the NWSZ and NSZ between c. 90 and 132 Ma, associated with Cretaceous Paleo-Pacific subduction. The largest intrusion is the c. 110 to 120 Ma Sepauk Tonalite. After collision of the East Java-West Sulawesi (Argo) Block, subduction ceased and post-collisional magmatism produced the c. 78 to 85 Ma Sukadana Granite and the A-type 72 Ma Sangiyang Granite in the SSZ. Rocks of the Pinoh Metamorphic Group mainly exposed in the NSZ, previously assumed to represent Paleozoic basement, contain abundant Early Cretaceous (110 to 135 Ma) zircons. They are interpreted as volcaniclastic sediments that formed contemporaneously with subduction-related volcanic rocks of the NSZ subsequently metamorphosed during intrusion of Cretaceous granitoids. There are no igneous rocks older than Cretaceous in the NSZ and older than Jurassic in the SSZ and there is no evidence for a continuation of a Triassic volcanic arc crossing Borneo from Sundaland to the east.

10 citations


Cites background from "Adakites without a slab: Remelting ..."

  • ...The youngest ages around 20 to 25 Ma belong to zircons probably derived from the Neogene Sintang Suite that intruded the Schwaner granitoids (Breitfeld et al., 2019)....

    [...]


Journal ArticleDOI
Abstract: Adakitic rocks are intermediate-acid magmatic rocks characterized by enrichment in light rare-earth elements, depletion in heavy rare-earth elements, positive to negligible Eu and Sr anomalies, and high La/Yb and Sr/Y ratios. Cenozoic adakitic rocks generated by partial melting of subducted oceanic crust (slab) under eclogite-facies conditions (i.e., the original definition of “adakite”) occur mainly in Pacific Rim volcanic arcs (intra-oceanic, continental, and continental-margin island arcs), whereas those generated by partial melting of thickened lower crust occur mainly in Tethyan Tibetan collisional orogens. In volcanic arcs, adakitic melts derived from the melting of subducted oceanic crust metasomatize the mantle wedge to form a unique rock suite comprising adakite-adakite-type high-Mg andesite-Piip-type high-Mg andesite-Nb-rich basalt-boninite. This suite differs from the basalt-andesite-dacite-rhyolite suite formed from mantle wedge metasomatized by fluids derived from subducted oceanic crust. Previously published data indicate that partial melting of mafic rocks can generate adakitic magmas under pressure, temperature, and hydrous conditions of 1.2–3.0 GPa, 800–1000°C, and 1.5–6.0 wt.% H2O, respectively, leaving residual minerals of garnet and rutile with little or no plagioclase. Cenozoic Au and Cu deposits occur proximally to adakitic rocks, with host rocks of some deposits actually being adakitic rocks. Adakitic rocks thus have important implications for both deep-Earth dynamics and Cu-Au mineralization/exploration. Although studies of Cenozoic adakitic rocks have made many important advances, there remain weaknesses in some important areas such as their tectonic settings, petrogenesis, magma sources, melt-mantle interactions of pre-Cenozoic adakitic rocks, and their relationship with the onset of plate tectonics and crustal growth. Future research directions are likely to involve (1) the generation of adakitic magmas by experimental simulations of partial melting of different types of rock (including intermediate-acid rocks) and magma fractional crystallization at different temperatures and pressures, (2) the relationship between magma reservoir evolution and the formation of adakitic rocks, (3) the tectonic setting and petrogenesis of pre-Cenozoic adakitic rocks and related geodynamic processes, (4) interactions between slab melts and the mantle wedge, (5) the formation of Archean adakitic tonalite-trondhjemite-granodiorite and its link to the onset of plate tectonics and crustal growth, and (6) the relationship between the formation of adakitic rocks and metal mineralization in different tectonic settings.

10 citations


Journal ArticleDOI
Abstract: Clastic sediments of Oligocene to Lower Miocene age form a major thick and widespread sequence in the Tatau-Nyalau province of the north Sarawak Miri Zone. New light and heavy mineral data, U-Pb detrital zircon geochronology and biostratigraphy are used to identify the age, depositional environment, and potential provenance of sediments to reconstruct the drainage evolution of NW Borneo. Based on the biostratigraphic ages, depositional environments and provenance characteristics we modify previous stratigraphy and divide the Oligocene to Lower Miocene sequences into the Nyalau Formation (Biban Sandstone Member and Upper Nyalau Member), Kakus Unit, and Merit-Pila Formation. Two dominant source provinces were identified: the Malay-Thai Tin Belt which supplied sediments dominated by Permian-Triassic zircons, and the Schwaner Mountains of central Borneo which are identified by abundant Cretaceous zircons. Sediments either came directly, or were recycled from older sedimentary rocks, from these sources. The Sunda River deposited the Nyalau Formation during the Oligocene to Early Miocene with a dominant Malay-Thai Tin Belt source. The Merit-Pila Formation of the Sibu Zone was deposited contemporaneously by a proto-Rajang River that drained Central Borneo (recycling the Rajang Group and Schwaner granitoids). Between c. 17 Ma the Sunda River system terminated and sedimentation was dominated by the northward prograding proto-Rajang River delta, depositing the Kakus Unit in the Miri Zone. This drainage system was active until the Late Miocene, before further uplift of Borneo terminated most sedimentation in the onshore part of present-day Borneo.

8 citations


Additional excerpts

  • ...11 and 14) reported by Hennig-Breitfeld (2019)....

    [...]


Journal ArticleDOI
Abstract: New field, geochemical, and geochronological data from the Segama Valley Felsic Intrusions (SVFI) of Sabah, north Borneo, shows them to be arc-derived tonalites; not windows or partial melts of a crystalline basement beneath Sabah. U-Pb zircon ages date emplacement in the Triassic and Jurassic: 241.1 ± 2.0 Ma, 250.7 ± 1.9 Ma, 178.7 ± 2.4 Ma, and 178.6 ± 1.3 Ma; contemporaneous with peaks in magmatism and detrital zircons in Sarawak and west Kalimantan (west Borneo). Isotopic data for Sr, Nd, and Pb from whole rocks, and for Hf and O from zircon all show mantle and/or MORB affinities indicating a mantle-derived origin. Enrichment of fluid mobile trace elements and trace element ratios indicate that the most likely setting for this is in a continuation of the Sundaland continental arc. There is no evidence in the field, geochemical, or zircon U-Pb data for continental basement in the Segama Valley region. The intrusive nature of the Segama Valley tonalites constrains the emplacement age of their supra-subduction zone host rocks to at least the Triassic. This new data expands the Triassic and Jurassic extent of Borneo and the Sundaland arc, and challenges models of Borneo's development predominantly through allochthonous terrane accretion in the Cretaceous. Instead, we propose a model of protracted autochthonous growth through supra-subduction zone crustal extension and associated magmatism.

8 citations


References
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01 Jan 1989
Abstract: Summary Trace-element data for mid-ocean ridge basalts (MORBs) and ocean island basalts (OIB) are used to formulate chemical systematics for oceanic basalts. The data suggest that the order of trace-element incompatibility in oceanic basalts is Cs ≈ Rb ≈ (≈ Tl) ≈ Ba(≈ W) > Th > U ≈ Nb = Ta ≈ K > La > Ce ≈ Pb > Pr (≈ Mo) ≈ Sr > P ≈ Nd (> F) > Zr = Hf ≈ Sm > Eu ≈ Sn (≈ Sb) ≈ Ti > Dy ≈ (Li) > Ho = Y > Yb. This rule works in general and suggests that the overall fractionation processes operating during magma generation and evolution are relatively simple, involving no significant change in the environment of formation for MORBs and OIBs. In detail, minor differences in element ratios correlate with the isotopic characteristics of different types of OIB components (HIMU, EM, MORB). These systematics are interpreted in terms of partial-melting conditions, variations in residual mineralogy, involvement of subducted sediment, recycling of oceanic lithosphere and processes within the low velocity zone. Niobium data indicate that the mantle sources of MORB and OIB are not exact complementary reservoirs to the continental crust. Subduction of oceanic crust or separation of refractory eclogite material from the former oceanic crust into the lower mantle appears to be required. The negative europium anomalies observed in some EM-type OIBs and the systematics of their key element ratios suggest the addition of a small amount (⩽1% or less) of subducted sediment to their mantle sources. However, a general lack of a crustal signature in OIBs indicates that sediment recycling has not been an important process in the convecting mantle, at least not in more recent times (⩽2 Ga). Upward migration of silica-undersaturated melts from the low velocity zone can generate an enriched reservoir in the continental and oceanic lithospheric mantle. We propose that the HIMU type (eg St Helena) OIB component can be generated in this way. This enriched mantle can be re-introduced into the convective mantle by thermal erosion of the continental lithosphere and by the recycling of the enriched oceanic lithosphere back into the mantle.

17,505 citations




Journal ArticleDOI
Abstract: Granites may be subdivided according to their intrusive settings into four main groups—ocean ridge granites (ORG), volcanic arc granites (VAG), within plate granites (WPG) and collision granites (COLG)—and the granites within each group may be further subdivided according to their precise settings and petrological characteristics. Using a data bank containing over 600 high quality trace element analyses of granites from known settings, it can be demonstrated using ORG-normalized geochemical patterns and element-SiO2 plots that most of these granite groups exhibit distinctive trace element characteristics. Discrimination of ORG, VAG, WPG and syn-COLG is most effective in Rb-Y-Nb and Rb-Yb-Ta space, particularly on projections of Y-Nb, Yb-Ta, Rb-(Y + Nb) and Rb—(Yb + Ta). Discrimination boundaries, though drawn empirically, can be shown by geochemical modelling to have a theoretical basis in the different petrogenetic histories of the various granite groups. Post-collision granites present the main problem of tectonic classification, since their characteristics depend on the thickness and composition of the lithosphere involved in the collision event and on the precise timing and location of magmatism. Provided they are coupled with a consideration of geological constraints, however, studies of trace element compositions in granites can clearly help in theelucidation of post-Archaean tectonic settings.

6,355 citations


Journal ArticleDOI
Abstract: Analytical data for Sr, Rb, Cs, Ba, Pb, rare earth elements, Y, Th, U, Zr, Hf, Sn, Nb, Mo, Ni, Co, V, Cr, Sc, Cu and major elements are reported for eocene volcanic rocks cropping out in the Kastamonu area, Pontic chain of Northern Turkey. SiO2% versus K2O% relationship shows that the analyzed samples belong to two major groups: the basaltic andesitic and the andesitic ones. High-K basaltic andesites and low-K andesites occur too. Although emplaced on continental type basement (the North Anatolian Crystalline Swell), the Pontic eocene volcanics show elemental abundances closely comparable with typical island arc calc-alkaline suites, e.g. low SiO2% range, low to moderate K2O% and large cations (Cs, Rb, Sr, Ba, Pb) contents and REE patterns with fractionated light and almost flat heavy REE patterns. ΣREE and highly charged cations (Th, U, Hf, Sn, Zr) are slightly higher than typical calc-alkaline values. Ferromagnesian elements show variable values. Within the basaltic andesite group the increase of K%, large cations, ΣREE, La/Yb ratio and high valency cations and the decrease of ferromagnesian element abundances with increasing SiO2% content indicate that the rock types making up this group developed by crystalliquid fractionation of olivine and clinopyroxene from a basic parent magma. Trace element concentration suggest that the andesite group was not derived by crystal-liquid fractionation processes from the basaltic andesites, but could represent a distinct group of rocks derived from a different parent magma.

3,822 citations