Tectonic Significance of a Mafic Conglomerat, North Island, New Zealand

C. Bender¹, K. M. Marsaglia¹, J. Vazquez¹, C. Mazengarb², and M. Marden^3
1California State University, Northridge, CA, ([email protected]), ([email protected]), ([email protected])
²Mineral Resources Tasmania, ([email protected])
³Landcare Research, New Zealand, ([email protected])

Localized outcrops of Miocene mafic conglomerate (Ihungia) occur near the frontal thrust of the West Coast Allochthon on the East Cape of North Island, New Zealand. At its northernmost extreme the thrust-based allochthon contains a Cretaceous ophiolitic section and Cretaceous to lower Miocene sedimentary units. The ophiolite is thought to have been emplaced during a complex reorientation of the subduction zone during Miocene time. The Ihungia Formation lies ~70 km north of the outcrops of ophiolite. Conglomerate clasts are predominantly equant to prolate (roller) in shape, suggesting at least a component of fluvial transport. Pebble counts reveal the conglomerate to be composed of approximately 60% mafic volcanic rocks, 30% intermediate to mafic plutonic/hypabyssal rocks, and 10% sedimentary rocks. Associated sandstone contains clasts of volcanic, plutonic, and amphibolite lithologies as well as detrital serpentine. The igneous clasts are predominantly calc-alkaline and tholeiitic basaltic andesite and gabbro. Their major and trace element composition are consistent with mafic to intermediate magmas that are generated in backarc basins or other suprasubduction settings.

AAPG Search and Discovery Article #90088©2009 Pacific Section Meeting, Ventura, California, May 3-5, 2009