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National News
Philippines:
Investigations of the oceanic lithospheric fragments in Mindoro:
Clues to the arc - continent collision in Central Philippines
G.P. Yumul Jr. 1,2 , C.B. Dimalanta 1 , K.L. Queaño 3 , E.J. Marquez 4 , R.A. Tamayo Jr. 1 , F.T. Jumawan 1 , L.T.
Armada 1 , J.A.S. Gabo 1 , R.D.O. Mediavillo 1 , D.E.L. Riguer 1 and C.D.U. Carranza 1
In 2003 researchers from the National Institute of Geological
Sciences, University of the Philippines, and the Mines and
Geosciences Bureau with support from the Department of Science
and Technology began a campaign to investigate the arccontinent
collision in west Central Philippines. The Palawan
microcontinental block, a fragment rifted from the southern
margin of mainland Asia, collided with the Philippine Mobile
Belt during the Early to Middle Miocene. As a result, various
features believed to have been produced by the collision event
were preserved and can be observed in Palawan, Romblon Island
Group (RIG), and western Panay (Fig. 1). The geological,
geophysical and geochemical investigations were initially carried
out in RIG and were followed by similar studies in northwestern
Panay. This year, investigations of the arc-continent
collision are currently being pursued in Mindoro Island. Of
particular interest are the northwest-southeast-trending ultramafic
belts in the island. Three ophiolitic belts have previously
been reported in Mindoro, namely: the Eocene Lubang-Puerto
Galera, the Cretaceous Mangyan and the Middle Oligocene
Amnay Ophiolitic Complex (Fig. 2). These belts of ophiolitic
rocks represent several episodes of accretion in the area.
The current research initiatives are directed at the collection of
geological, geochemical and geochronological data for the other
ophiolitic bodies (Mangyan and Puerto Galera Ophiolitic
Complexes) to see how these compare with the Amnay Ophiolitic
Complex (AOC). Are there really three separate ophiolite
complexes or are they all, in fact, part of one big dismembered
ophiolitic complex? Field campaigns in northwestern Mindoro
Island conducted in April 2008 began to tackle the various
aspects of the complex tectonic evolution of the area (Fig. 3).
During these surveys, extensive outcrops of the Halcon Metamorphic
Complex were identified in the northern part of the
island as well as in the Paluan area on the northwestern portion
of the island. In this area, the schists encountered consist
of talc-chlorite and quartz-mica varieties (Fig. 3a). The general
trends of the foliations of these metamorphic rocks were recorded.
In the westernmost portion of the island, talc-chlorite
schists exhibit foliations plunging to the northwest. A southeast-trending
plunge in the foliations of the schists near Paluan
was observed. In the northern coastal areas of Mindoro Island,
metamorphic rocks include quartz-mica schists, phyllites, amphibolites
and metagabbros which are best seen near the vicinity
of Abra de Ilog. Ophiolitic bodies were also observed
during the mapping at this site. Limited outcrops of the pillow
basalts, gabbros and peridotites of the Mangyan Ophiolitic
Complex were observed near Paluan. In Mamburao, the
limited exposures of the pillow basalts were highly weathered
and some show chloritization whereas the peridotites are moderately
to extensively serpentinized. Harzburgites representing
the upper mantle sequence of the AOC were mapped in Santa
Figure 1: Tectonic map of the Philippines showing subduction
systems bounding the archipelago. The Palawan microcontinental
block (yellow shaded region) is believed to have collided with the
Philippine archipelago during the Early Miocene. Features related to
the collision event are believed to be preserved in Palawan, Mindoro,
Romblon Island Group, and western Panay (light blue colored areas).
This year’s research campaign focused on Mindoro Island. Map created
using the Generic Mapping Tools (Wessel and Smith, 1995).
1
National Institute of Geological Sciences, University of the Philippines, Diliman, Quezon City, Philippines; 2 Department of Science and
Technology, Bicutan, Taguig City, Metro Manila, Philippines; 3 Mines and Geosciences Bureau, North Ave., Quezon City, Philippines;
4
College of Arts and Sciences, University of the Philippines, Manila, Philippines
InterRidge News 38 Vol. 17, 2008