Research ArticlePotassium-rich granite melt inclusions in zircon from gabbro-hosted felsic stringers, Mid-Atlantic Ridge at 13°34′N: E-MORB connection
Introduction
Minor leucocratic lithologies of broadly granitoid composition (‘felsic veins’, ‘oceanic plagiogranite’, or ‘trondhjemite’) regularly occur in plutonic assemblages of fast- to ultraslow-spreading mid-ocean ridges and ophiolites (e.g. Aumento, 1969; Coleman and Peterman, 1975; France et al., 2010; Kelley, 1997; Koepke et al., 2007; Silantyev et al., 2005). Typically, these rocks represent evolved low-K2O lithologies that range from diorite through tonalite, trondjhemite and, ultimately, low-K2O rhyolite (Brophy, 2009; Koepke et al., 2007, and references therein). The most extensive data on oceanic granitoid lithologies come from gabbroic assemblages of oceanic core complexes (OCCs), i.e., extensional structures that are typical of slow- and ultraslow-spreading ridges. For instance, drilling at the Mid-Atlantic (MAR) and South-West Indian (SWIR) ridges has demonstrated that granitoid veins commonly compose up to 1–1.5 vol% of dominantly gabbroic OCCs (Dick et al., 2000, Dick et al., 2017; Ildefonse et al., 2014). Although the volume of oceanic plagiogranite (OPG) is so low compared to basic rocks of oceanic crust, the nature of granitoid magmatism is one of the key questions for understanding processes of crustal accretion and lithosphere-hydrosphere interaction in spreading centers. The main models for the origin of oceanic granitoid magmatism are: high degree differentiation of MORB-type magma; partial melting of hydrothermally altered basic rocks; gabbro partial melting as a result of interaction with seawater-derived hydrothermal fluid (see reviews in Koepke et al., 2007 and Pietranik et al., 2017). The model of high degree differentiation of MORB-type magma is supported by regular association of OPG veins with earlier veins of evolved oxide gabbro within more primitive gabbroic wall rocks in dominantly gabbroic OCCs (Chen et al., 2019; Dick et al., 2017; Natland and Dick, 2002; Niu et al., 2002). The alternative model suggests that partial melting of gabbro triggered by a water-rich fluid phase is a common process in oceanic spreading ridges (e.g. Aranovich et al., 2010; Koepke et al., 2005; Silantyev et al., 2010; Wolff et al., 2013). Koepke et al. (2005) demonstrated that development of orthopyroxene and pargasite neoblasts in OCC gabbro (MAR, 23°N) is consistent with experimental partial melting caused by interaction with hydrous fluid at high temperature (900–1000 °C). The OPG bulk compositions show mostly low TiO2 levels that fit experimental hydrous melting of oceanic olivine gabbro rather than various experiments of MORB-type magma differentiation (Koepke et al., 2007). Quantitative estimates of the P-T-XH2O conditions of the OPG generation (Aranovich et al., 2010; Silantyev et al., 2010) as well as some geochemical features of zircon separated from the OPG (Aranovich et al., 2013a, Aranovich et al., 2017, Aranovich et al., 2020) have led the authors to conclude that the OPG formed due to interaction of gabbro with seawater-derived NaCl brines. According to different trace element trends and δ18O in zircon Pietranik et al. (2017) suggested that both OPGs and oxide gabbros may have originated in two alternative ways within the same OCC gabbro section in the SWIR: (1) differentiation of mantle magma and (2) ocean crust hydrous partial melting.
The characteristic major-element compositional features of OPGs: low K2O and TiO2, and high FeO/MgO ratio – have been retrieved from bulk rock analysis (e.g. Aranovich et al., 2013a; Coleman and Peterman, 1975; Koepke et al., 2007). Few analyses of unheated melt inclusions in zircon separated from OPG showed compositions broadly similar to the host, but with somewhat elevated K2O (Aranovich et al., 2013a; Grimes et al., 2009). Nozaka et al. (2019) showed indirect signs of elevated K2O in felsic veins from the Atlantis Bank OCC, as the veins associate with biotite development in the host gabbro. First direct data on the oceanic granitoid melt composition were obtained on a re-melted micro-inclusion from MAR (13°30′N OCC) (Aranovich et al., 2015). The microprobe analysis showed unexpectedly high K2O and appreciable amounts of H2O and Cl (Aranovich et al., 2015) – the features strongly different from bulk composition of most OPGs. This preliminary result prompted us to pay attention to the geochemically anomalous MAR magmatism near 14°N (Bougault et al., 1988; Dosso et al., 1991; Hémond et al., 2006; Wilson et al., 2013; Yang et al., 2018). In this work we report on the new finds of K2O-rich silicic melt inclusions in zircon from felsic veins hosted by the 13°30′N OCC gabbronorite and on a range of basalt melt enrichment in spatially associating lavas, and discuss a possible direct link between E-MORB type magma and high‑potassium granitoid melt. Mineral abbreviations used in the text and figures are as follows: Ap, apatite; Bt, biotite; Crd, cordierite; Cpx, clinopyroxene; Hbl, hornblende; Ilm, ilmenite; K-Fsp, Potassium feldspar; Mu, muscovite; Opx, orthopyroxene; Pl, plagioclase; Qz, quartz; Tnt, titanite; Zrn, zircon.
Section snippets
Electron microprobe analyses (EMPA)
Local analyses of minerals and silicate glasses were performed using a JEOL Superprobe 8200 equipped with five wave-length dispersive spectrometers in the Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry (Moscow). Operating conditions: accelerating voltage 15–20 kV; beam current 20 nA for minerals and 10 nA for glasses; focused beam spot diameter 1 μm for minerals and 3 to 10 μm for glasses. ZAF correction was applied for the average atomic number, absorption, and
Regional features
First order segment of the Mid-Atlantic Ridge between the Fifteen Twenty (15°19′ N) and Marathon (12°37′ N) fracture zones (Fig. 1) consists of several second order segments including one with a typical symmetric development of basalt cover (13°48′–14°24′ N) and the other second order segments dominated by asymmetric tectonic extension with sporadic volcanism (Smith et al., 2008 and references therein). The tectonic extensional settings of the region resulted in exposing a number of
Basalts
The collected basalts are represented by pillow lavas that often retain fragments of highly fractured external layers of quenched glass up to 20 mm thick. The groundmass types show quenched plagioclase-free pyroxene-olivine hyalopilitic to pilotaxitic textures with skeletal olivine crystals (Supplementary Fig. S1 a–c), pilotaxitic to microdolerite texture with elongated plagioclase microcrysts (Supplementary Fig. S1 d–f), and variolitic texture (Supplementary Fig. S1 g). The collected lavas
Basalt glasses
Chemical composition of 28 basalt glasses (Supplementary Table S2) shows a wide range in both major and trace elements (Fig. 7, Fig. 8). To reveal possible heterogeneity of magmatic source we use K2O/TiO2 ratio in the glasses, since K and Ti are, respectively, highly and moderately incompatible to mantle peridotite, whereas K2O/TiO2 ratio in basalt magma cannot be seriously modified by plagioclase + olivine dominated crystal fractionation (e.g., Devey et al., 1994). The studied basalt lavas
Inclusions versus stringer modal composition
To verify that the measured composition of the small homogenized melt inclusions in zircon (Fig. 14) would crystallize into the assemblage found in the larger exposed inclusion shown in Fig. 13, we calculated equilibrium phase diagram for the average melt composition from Table 1. Calculations employed DOMINO software (De Capitani and Petrakakis, 2010) along with the Holland and Powell (2011) database for both solids (including solid solution models) and melt. The calculations returned complete
Conclusions
Crystallized, unusually enriched in K2O inclusions have been revealed in zircon from felsic stringers hosted by gabbronorite. At the temperature close to 800 °C and estimated pressure about 1.5 kbar the inclusions completely homogenize into a K2O-rich (3.9 ± 0.1 wt%), H2O-bearing (about 4 wt%) granite (SiO2 70 ± 2 wt%) melt. Exposed well-crystallized inclusion consists of the muscovite + biotite + plagioclase + K-feldspar + quartz assemblage. The inclusions could not represent either residual
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
Constructive reviews by Qiang Ma and anonymous reviewer of the earlier draft of manuscript are appreciated.
Financial support was provided by the RF Ministry of Science and Education under agreement # 075-15-2020-802. Expedition works were financed by the Federal Agency on Nature Management, RF Ministry of Natural Resources and Ecology.
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