The geochemistry of volcanic, plutonic and turbiditic rocks from Sumba, Indonesia

Abstract

Rocks that reveal the geology of Sumba for times before the Later Miocene (∼16 Ma) are relatively few and are not particularly well exposed. This has led to uncertainty about the nature of the basement rocks of the island and especially about whether Sumba originated as a fragment of Australia, or of that part of southeastern Eurasia which many authors have called Sundaland. A third possibility is that Sumba is underlain by arc material generated on the ocean floor and is not a fragment of either continent. We have studied the geochemistry of volcanic, plutonic and turbiditic rocks collected from Sumba in an attempt to provide additional insight into the island's origin and history between Late Cretaceous (∼86 Ma) and Early Miocene (∼16 Ma) times. Late Cretaceous to Early Oligocene (∼31 Ma) volcanic rocks on Sumba range compositionally from basalts to andesites, and are of typical oceanic island-arc affinity, exhibiting geochemical characteristics similar to those of high-Al basalts and their derivatives. Compositions indicate evolution along both calc-alkaline and tholeiitic trends. Some samples show indications of possible modifications by slab-derived melts and/or related fluids and also of contamination by turbiditic sediments. Gabbros and diorites collected from the Paleocene Tanadaro intrusion are compositionally similar to the associated volcanic rocks and, we consider, represent the plutonic equivalents of high-Al basalt. The geochemistry of Cretaceous turbiditic sedimentary rocks on Sumba indicates close proximity to an intra-oceanic island-arc environment. These results are consistent with the geochemical, sedimentological, stratigraphic, paleontological and paleomagnetic results of other investigators which together indicate that: (1) Late Cretaceous to Early Oligocene volcanic, plutonic and volcaniclastic rocks of Sumba are island-arc- and forearc-related; (2) the arc involved appears to have been what we refer to as the Great Indonesian Volcanic Arc, which had been active in this area from at least ∼86 to approximately ∼31 Ma. The Great Indonesian Volcanic Arc was closely affiliated with, but may have occurred offshore Sundaland. Sumba is therefore a fragment of this oceanic (Aleutian-type) island arc and not a piece of the main Sundaland continent. The geochemistry of the igneous rocks on Sumba is inconsistent with what we would expect from an Andean-type Continental Arc. No continental basement is required to explain the composition and origin of the igneous rocks of Sumba.