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

TL;DR: In this paper, geochronological and geochemical data for Neogene magmatism from West Sarawak were presented, showing that 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 basaltic rocks in the crust.

About: This article is published in Lithos.The article was published on 2019-11-01 and is currently open access. It has received 22 citations till now. The article focuses on the topics: Adakite & Mafic.

Summary

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.