Back-arc basin basalt systematics

Abstract

The Mariana, east Scotia, Lau, and Manus back-arc basins (BABs) have spreading rates that vary from slow ( 6 50 mm/yr) to fast ( s 100 mm/yr) and extension axes located from 10 to 400 km behind their island arcs. Axial lava compositions from these BABs indicate melting of mid-ocean ridge basalt (MORB)-like sources in proportion to the amount added of previously depleted, water-rich, arc-like components. The arc-like end-members are characterized by low Na, Ti and Fe, and by high H 2 O and Ba/La; the MORB-like end-members have the opposite traits. Comparisons between basins show that the least hydrous compositions follow global MORB systematics and an inverse correlation between Na8 and Fe8. This is interpreted as a positive correlation between the average degree and pressure of mantle melting that reflects regional variations in mantle potential temperatures (Lau/Manus hotter than Mariana/Scotia). This interpretation accords with numerical model predictions that faster subduction-induced advection will maintain a hotter mantle wedge. The primary compositional trends within each BAB (a positive correlation between Fe8, Na8 and Ti8, and their inverse correlation with H 2 O(8) and Ba/La) are controlled by variations in water content, melt extraction, and enrichments imposed by slab and mantle wedge processes. Systematic axial depth (as a proxy for crustal production) variations with distance from the island arc indicate that compositional controls on melting dominate over spreading rate. Hydrous fluxing enhances decompression melting, allowing depleted mantle sources just behind the island arc to melt extensively, producing shallow spreading axes. Flow of enriched mantle components around the ends of slabs may augment this process in transform-bounded back-arcs such as the east Scotia Basin. The re-circulation (by mantle wedge corner flow) to the spreading axes of mantle previously depleted by both arc and spreading melt extraction can explain the greater depths and thinner crust of the East Lau Spreading Center, Manus Southern Rifts, and Mariana Trough and the very depleted lavas of east Scotia segments E8/E9. The crust becomes mid-ocean ridge (MOR)-like where the spreading axes, further away from the island arc and subducted slab, entrain dominantly fertile mantle. ß

Arenigian tholeiitic basalts in the Famatina Ordovician basin, northwestern Argentina: emplacement conditions and their tectonic significance.

“…Such features may be compared with basalts emplaced along arc-backarc basins systems, where the lava compositions are indicative of mid-ocean ridge basalt (MORB)-like sources with high water contents (cf. Taylor and Martínez, 2003). A depleted mantle source for the magma is also consistent with the observed Nb/Yb ratio displayed for the studied basalts, 1.34 in average (cf.…”

Os isotopic evidence for a carbonaceous chondritic mantle source for the Nagqu ophiolite from Tibet and its implications

“…The Nagqu ophiolite is a fragment of juvenile oceanic crust formed in the Early Jurassic and preserved during the later collision between the Amdo microterrane and Lhasa terrane. The consistency between the Osisotope characteristics of the Nagqu ophiolite and the convecting asthenospheric mantle indicates the ophiolite formed in a mature back-arc basin, where both arc-like and MORBlike basalts are generated [86]. Thus, the Tibetan lithosphere is primarily derived from remnants of Tethyan oceanic lithosphere, formed by the transformation of the normal asthenospheric mantle in the Mesozoic.…”

The dolerite dyke swarm of Mongo, Guéra Massif (Chad, Central Africa): Geological setting, petrography and geochemistry

“…Mongo dolerite compositions are very distinct from typical arc-type tholeiite as proposed by Jakeš and Gill [44], shown in Th-Hf-Ta [45] and Th-Tb-Ta diagrams [17]. Accordingly, the arc-type features [46,47] exhibited by some Mongo dolerites should be related to the addition of terrigenous sediments in mantle source, as proposed by Wayne [38] for Huronian low-Ti continental tholeiites.…”

“…High Ba/Nb (31-74) and Th/Ta (2-5) evidence an orogenic context [46,47], also shown in the Th-Tb-Ta diagram ( [17], Figure 6A). Incompatible elements (Table 1 and Figure 7A) exhibit negative anomalies in Nb and Ta and differ from positive anomalies in Initial Rift Tholeiite (IRT, [25]), thus preventing a relationship with a rifting environment (Figure 11).…”