Geochemistry of Lau Basin volcanic rocks: influence of ridge segmentation and arc proximity

Abstract

Side-scan sonar imaging of the Central Lau Basin (SW Pacific) has revealed a Central Lau Spreading Centre (CLSC) propagating southwards at the expense of an Eastern Lau Spreading Centre (ELSC) with a small Intermediate Lau Spreading Centre (ILSC) forming a ‘relay’ between the two. Volcanic rocks sampled along these spreading centres, and from two adjacent seamounts, are glassy to fine-grained pillow lavas and sheet flows of basalts, ferrobasalts and andesites. The evolved rocks are mostly confined to the propagating tip of the CLSC and can be explained by a high rate of cooling relative to magma supply, as invoked for magma genesis at propagating ridges elsewhere. Compared with equivalent rocks from the eastern Pacific, the most evolved members of the CLSC suite require similarly high degrees (>90%) of fractional crystallization from their basaltic parents. Fractional crystallisation cannot, however, account for the compositional differences between CLSC and ELSC lavas. Whereas the composition of the CLSC lavas lies just within the compositional spectrum of typical N-MORB, the ELSC lavas are distinctly enriched in alkali and alkaline earth elements, reach oxide and apatite saturation at lower Fe, Ti and P concentrations and generally show greater vesicularity despite slightly greater depths of eruption, all indicative of a water-rich subduction component. They also have lower contents of Ni, Sc, Na and Fe and higher contents of Si at a given MgO concentration that indicate a more depleted and more hydrous mantle beneath the ELSC compared with the CLSC. These results provide further evidence that, beneath the Central Lau Basin, the source composition changes progressively from MORB-type to island arc tholeiite type as the subduction zone is approached, both eastwards from the CLSC to ELSC and southwards along the ELSC to the Valu Fa Ridge. They also indicate that the composition of the subduction component may vary systematically away from the arc, with Th, LREE, Ba, Rb and H (as H 2 O) all present close to the arc, only Ba, Rb and H 2 O present at intermediate distances and just H 2 O perceptible at the furthest distances.

Back-arc basin basalt systematics

“…[34]). Samples from the arc-proximal and shallow Valu Fa Ridge (VFR) and southern East Lau Spreading Center (sELSC) have the lowest Na8 and Ti8, and the highest Ba/La and H 2 O(8) (Figs.…”

Mesoproterozoic high Fe–Ti mafic magmatism in western Shandong, North China Craton: Petrogenesis and implications for the final breakup of the Columbia supercontinent

“…Although high degrees of fractionation (65-73%) of olivine, clinopyroxene and plagioclase have been reported for high Fe-Ti basaltic rocks (Clague and Bunch, 1976;Clague et al, 1981;Perfit and Fornari, 1983), and experimental studies have shown that broadly basaltic compositions are still preserved after 90% fractionation (Dixon and Rutherford, 1979), the derivative basaltic magma is generally tholeiitic in composition, as is the case at propagating spreading centers along mid-ocean ridges (Clague et al, 1981;Perfit and Fornari, 1983;Sinton et al, 1983) or in back-arc basins (Pearce et al, 1994;Parson and Wright, 1996;Lagabrielle et al, 1997). Clearly, for the Yishui high Fe-Ti gabbros in this study, fractional crystallization alone fails to explain the Fe-Ti enrichment, coupled with an alkaline composition.…”

“…Excluding the oceanic setting, such as propagating spreading centers at mid-ocean ridges (Clague et al, 1981;Perfit and Fornari, 1983;Sinton et al, 1983) and in back-arc basins (Pearce et al, 1994;Harper, 2003), several studies have demonstrated that high Fe-Ti basic igneous rocks are also generated in continental settings, such as in the Afar Rift of Ethiopia (Barberi et al, 1975) and in the northern and southern sections of the Red Sea Rift (Juteau et al, 1983;Cocherie et al, 1994). Moreover, most of the Fe-Ti-rich compositions are found in plutonic environments and only rare cases are known for erupted rocks (Clague et al, 1981;Perfit and Fornari, 1983;Sinton et al, 1983;Hunter and Sparks, 1987;Pearce et al, 1994;Hooper, 2000;Harper, 2003;Zhang et al, 2012).…”

“…Excluding the oceanic setting, such as propagating spreading centers at mid-ocean ridges (Clague et al, 1981;Perfit and Fornari, 1983;Sinton et al, 1983) and in back-arc basins (Pearce et al, 1994;Harper, 2003), several studies have demonstrated that high Fe-Ti basic igneous rocks are also generated in continental settings, such as in the Afar Rift of Ethiopia (Barberi et al, 1975) and in the northern and southern sections of the Red Sea Rift (Juteau et al, 1983;Cocherie et al, 1994). Moreover, most of the Fe-Ti-rich compositions are found in plutonic environments and only rare cases are known for erupted rocks (Clague et al, 1981;Perfit and Fornari, 1983;Sinton et al, 1983;Hunter and Sparks, 1987;Pearce et al, 1994;Hooper, 2000;Harper, 2003;Zhang et al, 2012). Some of these intrusions host world class Fe-Ti oxide deposits, such as the Skaergaard Intrusion in East Greenland (e.g., Nielsen, 2004;Jakobsen et al, 2005Jakobsen et al, , 2011, the Sept Iles layered intrusion in Canada (e.g., Namur et al, 2011), the Bushveld Complex in South Africa (e.g., VanTongeren and Mathez, 2012), the Panzhihua intrusive complex in southwestern China (e.g., Zhou et al, 2005Zhou et al, , 2008a and the Damiao complex in North China (e.g., Zhao et al, 2009).…”

“…) and back-arc spreading centers (Christie and Sinton, 1981;Pearce et al, 1994) in oceanic environments, and to rift (Barberi et al, 1975;Juteau et al, 1983;Cocherie et al, 1994) and mantle plume (Ripley et al, 1998;Duchesne and Charlier, 2005;Higgins, 2005;Upton et al, 2005;Zhou et al, 2005;Tegner et al, 2006;Namur et al, 2010) settings in the continental regime. In view of the fact that all high Fe-Ti gabbros were intruded into ∼2.55 Ga TTG gneisses, this implies a continental environment for their generation in western Shandong.…”