Adakites in the Truong Son and Loei fold belts, Thailand and Laos: Genesis and implications for geodynamics and metallogeny

Highlights

• New ages for the Truong Son (ca. 305 Ma) and Loei (ca. 240 Ma) adakites

• The adakite formation was coeval with the associated Cu–Au mineralization.

• The adakites may have formed during local subduction initiation.

Abstract

LA-ICP-MS U–Pb zircon dating reveals that the Phu Kham adakites in the Truong Son Belt were emplaced during the Late Carboniferous (ca. 306 to 304 Ma), whereas the Puthep 1 (PUT 1) adakites in the Loei Fold Belt were formed during the Middle Triassic (ca. 244 to 241 Ma). These rock formation ages are largely coeval to Re–Os molybdenite ages (Phu Kham: ca. 304 Ma; PUT 1: ca. 246 Ma), suggesting close temporal links between adakite formation and copper–gold mineralization. New geochemical results show that the Phu Kham and PUT 1 adakites are characterized by low HREE and Y contents, but elevated LREE, Sr, Sr/Y, and La/Yb values. In addition, the two adakites have relatively high ¹⁴³Nd/¹⁴⁴Nd ratios (0.512648 to 0.512719), Mg# (Phu Kham: 37–68; PUT 1: 40–65), and Cr and Ni contents. These results are best interpreted as representing slab melts that have interacted with supra-subduction zone mantle wedge during ascent. Tectonically, the Phu Kham and PUT 1 adakites were most likely formed during the initiation of subduction of the Ailaoshan–Song Ma and the Main Paleo-Tethys ocean plates respectively. Close temporal relations between the adakite formation and the copper–gold mineralization suggest strong genetic links between the two in the mainland SE Asia region.

Introduction

Adakite is a long-standing controversial topic in igneous petrology, because of the diverse hypotheses on the origins and formational mechanisms of these rocks (e.g., Moyen, 2009). Adakites are intermediate to felsic volcanic or intrusive rocks that are genetically different from calc-alkaline rocks, and are recognized based mostly on trace element characteristics. Adakites show HREE depletions (Yb ≤ 1.9 ppm) and have high Sr (> 400 ppm), but low Y (≤ 18 ppm) contents, resulting in high Sr/Y (≥ 20) and La/Yb (≥~ 8) ratios (e.g., Defant and Drummond, 1990, Drummond and Defant, 1990, Martin, 1999, Martin et al., 2005). These geochemical signatures are commonly interpreted to be the result of garnet involvement in their petrogenesis. However, such involvement may also be achieved via many other petrogenetic processes, including: 1. Partial melting of subducted oceanic crusts (Kay, 1978, Stern and Futa, 1982, Martin, 1987, Defant and Drummond, 1990, Kay et al., 1993, Stern and Kilian, 1996, Sigmarsson et al., 1998, Zhou et al., 2006, Falloon et al., 2008, Lazaro and Garcia-Casco, 2008, Ickert et al., 2009, Eyuboglu et al., 2013a, Eyuboglu et al., 2013b); 2. Amphibole and garnet fractionation from hydrous basalts at either low or high pressures (Castillo et al., 1999, Macpherson et al., 2006, Rodriguez et al., 2007, Alonso-Perez et al., 2009, Chiaradia et al., 2009, Li et al., 2009, Coldwell et al., 2011, Gao et al., 2011, Rooney and Franceschi, 2011, Tiepolo et al., 2011); 3. Partial melting of thickened lower continental crust via basaltic underplating (Petford and Atherton, 1996, Wen et al., 2008, Zhao et al., 2008, Goss and Kay, 2009, Zhao et al., 2009, Hastie et al., 2010, Yuan et al., 2010, He et al., 2011, Zeng et al., 2011, Yu et al., 2012) or delamination (Gao et al., 2004, Wang et al., 2006, Wang et al., 2007, Huang et al., 2008, Xu et al., 2008, Xu et al., 2012); and 4. Slab-derived melt–mantle peridotite interaction (Bourdon et al., 2002, Samaniego et al., 2005, Gao et al., 2007, Konig et al., 2007, Wang et al., 2008, Manikyamba et al., 2009, Zhu et al., 2009, Batkhishing et al., 2010, Gao et al., 2010, Samaniego et al., 2010, Ayabe et al., 2012, J. Liu et al., 2012, Mao et al., 2012, Y. Liu et al., 2012).

Currently, much research has been attempted to address the question of what geodynamic processes may have triggered adakite formation, of which some of the hypotheses include:

• Subduction of young and hot oceanic crusts (e.g., Drummond and Defant, 1990, Martin, 1999);

• Subduction initiation (e.g., Sajona et al., 1993, Peacock et al., 1994);

• Arc collision (e.g., Sajona et al., 2000);

• Flat subduction (e.g., Gutscher et al., 2000, Beate et al., 2001);

• Fast and oblique subduction (e.g., Yogodzinski et al., 1995);

• Ridge subduction (e.g., Kay et al., 1993);

• Slab window (e.g., Ickert et al., 2009);

• Slab tearing following ridge–trench collision (e.g., Pallares et al., 2007);

• Continental collision (e.g., Gao et al., 2004).

Due to the many proposed petrogenetic models and geodynamic mechanisms for adakites, caution must be taken in using the term adakite in a genetic sense (e.g., Castillo, 2012). Therefore, the term adakite used here is a descriptive term referring to the rocks with the unique adakititic geochemical signals (e.g., HREE depletions and high Sr concentrations, leading to high Sr/Y and La/Yb), regardless of its genesis and settings.

Another unresolved and still hotly debated issue is whether adakites are genetically related to copper–gold mineralization formed either in subduction or post-subduction tectonic settings (e.g., Thieblemont et al., 1997, Sajona and Maury, 1998, Beate et al., 2001, Oyarzun et al., 2001, Mungall, 2002, Oyarzun et al., 2002, Richards, 2002, Borisova et al., 2006, Wang et al., 2006, Richards and Kerrich, 2007, Chiaradia et al., 2009, Richards, 2009, Shafiei et al., 2009, Xianghua et al., 2009, Jégo et al., 2010, J. Liu et al., 2012, Y. Liu et al., 2012, Hou et al., 2011, Richards, 2011a, Richards, 2011b, Richards, 2011c, Sun et al., 2011a, Sun et al., 2011b, Castillo, 2012, Jiang et al., 2012, Richards et al., 2012, Wang et al., 2012). Of particular interest for this study are porphyry copper deposits in the mainland SE Asia. In subduction zones, porphyry copper deposits are thought to be generated from hydrothermal fluids exsolved from the cooling of H₂O-rich, subduction-related magmas derived from a metasomatised mantle wedge (calc-alkaline) or from slab melts (adakite) (e.g., Richards, 2003, Richards, 2011a, Cooke et al., 2011). In contrast, calc-alkaline to alkaline magmas associated with porphyry deposits within post-subduction tectonic setting (e.g., arc collision, continent–continent collision, subduction reversal) is predominantly derived from partial melting of subduction-modified sources in the upper plate lithosphere (e.g., Richards, 2009, Richards, 2011a). Both scenarios yield a hot oxidized and hydrous mafic melt enriched in Cl, S, and metals (Mungall, 2002, Wang et al., 2006, Kelley and Cottrell, 2009, Richards, 2009, Shafiei et al., 2009, Hou et al., 2011, Jiang et al., 2012, Prouteau and Scaillet, 2013).

A diverse suite of calc-alkaline, adakitic to alkaline magmas has therefore the ability to concentrate metals at the magmatic stage. Correlations between these three magmas and ore-forming processes have been studied (Richards, 1990, Blevin and Chappell, 1992, Hedenquist and Lowenstern, 1994, Keith et al., 1997, Hattori and Keith, 2001, Oyarzun et al., 2001, Mungall, 2002, Richards, 2003, Borisova et al., 2006, Wang et al., 2006, Richards and Kerrich, 2007, Richards, 2009, Jégo et al., 2010, Jenner et al., 2010, Hou et al., 2011, Richards, 2011a, Richards, 2011c, Jiang et al., 2012, Richards et al., 2012, Prouteau and Scaillet, 2013). Nevertheless, the fact that not all magmas generated in the convergent or collisional fronts will yield fertile melts forming fertile metallogenic environments indicates that additional petrogenetic processes may be required to destabilize mantle sulfides and release chalcophile and siderophile elements (e.g., Cu–Au) into the melts (Mungall, 2002, Richards, 2003, Jégo et al., 2010, Jenner et al., 2010, Richards, 2011a).

In case of slab melt-derived adakitic magmas, very little research has been attempted to define the genetic relations between the slab melts and ore depositional processes. Oyarzun et al. (2001) and Mungall (2002) have proposed that slab melts are unusually more oxidized and more H₂O- and SO₂-rich than mantle-derived melts. They argued that such ferric-rich melts may have more potential to cause high oxidation in the mantle wedge, leading to complete sulfide destruction and subsequent partitioning of chalcophile and siderophile elements into hybridized slab melts. In contrast, Wang et al. (2007) have argued that slab melts may have low fO₂ that may lower the fO₂ of the mantle wedge. Thus, genetic connections between adakite and copper–gold mineralization remain controversial and more work is required before any link can be established.

In this paper, we report geochronological, geochemical and Sr–Nd isotopic data for the newly identified adakites from the Phu Kham (central Laos) and Puthep 1 (PUT 1; northeast Thailand) deposits (Fig. 1C) in the Truong Son and Loei fold belts, respectively. These rocks are closely spatially and temporally linked to porphyry-related skarn Cu–Au systems (Fig. 2). Thus, our study focuses on the geochemical and isotopic characteristics of these rocks to elucidate possible links between adakitic magmas and metallogenesis in the region.