New Backarc Magma Type: Vate Trough, New Hebrides
Abstract
The Vate Trough is one of three en echelon, nascent backarc basins located about 50 km to the east of the volcanic front comprising the southern New Hebrides Island Arc. These basins have developed in a left-lateral transtensive stress regime. During two research voyages using the Australian Marine National Facility (RVs Franklin and Southern Surveyor) in 2001 and 2004 respectively, samples from extensive, young, pillowed and sheet lava flows, erupted from numerous fissures and a caldera, were recovered. Hydrothermal activity was detected in 2001 in the caldera on the summit region of the Nifonea Ridge (McConachy et al., 2005) The volcanic rocks forming the floor of the Vate Trough and Nifonea Ridge are basalt, trachybasalt and basaltic trachyandesite. The two latter rock types are more commonly found in intra-continental rifts with total alkalies in excess of 6 wt%\. All samples are highly oxidized with fO2 4 log10 orders above FMQ. On the basis of major and trace element geochemical compositions, three major rock types are identified: a light rare earth element (LREE)-enriched group with chondrite-normalised (CN) La/Yb ~3; a group with lesser CN La/Yb fractionation (~2) but overall elevated REE at about 70- to 30-times chondritic and small negative Eu anomalies; and a group with unfractionated CN La/Yb at ~15-times chondritic. The remarkable feature of the Vate Trough suite is the absence of a subduction zone signature despite the proximity of the Trough to the arc volcanic front. For example, there are distinct negative "spikes" of Pb relative to Ce, Sr relative to Nd, and elevated Nb (~17 ppm) relative to U (~0.5 to 0.7 ppm) similar to many ocean island basalts and in stark contrast with typical island arc basalts. The combined Sr-Nd-Pb isotopic geochemistry of the Vate Trough suite is consistent with an "Indian Ocean" character: 87Sr/86Sr ranging from 0.703205 to 0.703301; 143Nd/144Nd from 0.513099 to 0.512978; and 207Pb/204Pb at specific 206Pb/204Pb elevated above the Northern Hemisphere Reference Line. These types of isotopic characteristics are typical of most of the backarc basins in the western Pacific, and have been identified in the mantle wedge source of the basaltic volcanoes forming the central region of the New Hebrides Arc lavas adjacent to the collision zone of the subducting D'Entrecasteaux Ridge. It is likely that this Indian Ocean mantle source component has been trapped under much of the North Fiji Basin following subduction polarity reversal in the Late Miocene, and current Australian-Indian Plate subduction beneath the Pacific Plate.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- Bibcode:
- 2006AGUFM.V34C..06A
- Keywords:
-
- 8413 Subduction zone processes (1031;
- 3060;
- 3613;
- 8170);
- 8415 Intra-plate processes (1033;
- 3615)