Tags

, , , ,

ResearchBlogging.org
Around 45 Million years ago (Eocene), the South China sea started opening.  With that movement, what is now Palawan and Busuanga coming from mainland Asia (continental origin) , drifted toward what is now the Luzon island (see this).  Eventually (after again millions of years), their bases bumped with each other forming what we now see in our maps today.

Mindoro is in between the Palawan-Busuanga block, and the Luzon Island. Geologists in their previous studies suggest that at least the south western Mindoro island came with the Palawan and Busuanga block.  However, there are debates regarding the origin of the rest of the island.  One model says that the rest of the island comes from the Philippine Mobile belt (where Luzon island is a part of) while another model says that the whole island has a mainland Asian origin.

In their recent paper [1], geologists investigated petrographic and geochemical attributes of sediments from the Late Eocene Lasala Formation to find a more definite answer. This formation is the oldest sedimentary sequence in northwestern Mindoro. 

The authors concluded that Mindoro is a fragment of Mainland Asia similar to Palawan and Busuanga.

Morever, they said that the easternmost boundary of the collision between the Asian continental block (Palawan, Busuanga, and now Mindoro) and the Philippine Mobile Plate is in between Sibuyan island and Masbate island (as suggested by other studies also). The whole Romblon Group of Islands therefore, is part of the continental block. This fact is particularly interesting and is connected to my next post on gekko colonialization of the Philippines.

Read the abstract and some data and analyses from the paper below:

Petrography, geochemistry, and tectonics of a rifted fragment of Mainland Asia: evidence from the Lasala Formation, Mindoro Island, Philippines

R. A. B. Concepcion [1], C. B. Dimalanta [1], G. P. Yumul Jr [1,2], D. V. Faustino-Eslava [1], K. L. Queaño [3,4], R. A. Tamayo Jr [1], and A. Imai [5,6]

[1] Rushurgent Working Group—Tectonics and Geodynamics Group, College of Science, National Institute of Geological Sciences, University of the Philippines, Diliman, Quezon City, Philippines
[2] Department of Science and Technology, Bicutan, Taguig City, Philippines
[3] Mines and Geosciences Bureau—Central Office, Department of Environment and Natural Resources, North Avenue, Diliman, Quezon City, Philippines
[4] Earth and Materials Science and Engineering Department,Mapua Institute of Technology, Intramuros, Manila, Philippines
[5] Department of Earth Resources Engineering, Kyushu University, Fukuoka, Japan
[6] Department of Earth Science and Technology, Akita University, Akita, Japan

Island of Mindoro. Courtesy: Google Earth, 2012.

Abstract:

Petrological and geochemical investigations of the sedimentary Lasala formation in Northwest Mindoro, Philippines, offer new insights into the origin of this geologically contentious region. Mindoro island’s position at the boundary between Sundaland and the Philippine Mobile Belt has led to variable suggestions as to how much of it is continent derived or not. The Eocene Lasala formation overlies the Jurassic Halcon metamorphics, a regionally metamorphosed suite generally thought to have formed as a result of arc-continent collision processes. The sedimentary formation consists mainly of sandstones and shales interbedded with mudstones, basalt flows, and subordinate limestones and conglomerates. Petrographic information on the Lasala clastic rocks demonstrates a uniform framework composition that is predominantly quartzose. Major oxide, trace element abundances, and various elemental ratios similarly impart a strongly felsic signature. These characteristics are taken to indicate a chiefly continental, passive margin derivation and deposition of the Lasala sediments during the Eocene. The weak indication of active margin influence is suggested to be an inherited signature, supported by paleogeographic models of the southeastern Asian margin area during the pre- Cenozoic.

Some supporting data and analyses from the paper.

Unfortunately, I could not put all the data and plots from the paper. Gallery view is after the reference.

Function diagrams:

Function diagrams proposed by (A) Bhatia (1983) and (B) Roser and Korsch(1988) show that the quartzose sedimentary and felsic igneous provenance for the Mindoro clastic rocks (black square) are grouped with the Buruanga (stripes), Palawan (black), and Nanxiong (white) fields while the Baguio (gray) field do not overlap with the rest. Courtesy: R. A. B. Concepcion et al, International Journal of Earth Sciences 101, (2012).

The K2O/Na2O versus SiO2 diagram:

When plotted on the K2O/Na2O versus SiO2 diagram, the Mindoro samples mostly fall within the active continental margin (ACM) and passive margin (PM) tectonic settings together with the fields for Nanxiong, Palawan, and Buruanga. However, samples from Baguio (Klondyke Formation) are easily distinguished and fall within the arc field. Courtesy: R. A. B. Concepcion et al,International Journal of Earth Sciences 101, (2012).

Whole-rock rare earth element (REE) patterns:

Top: Chondrite-normalized multi-element plot of the Lasala samples (black square). Note the dip in the element Eu (Europium) is consistent with derivation from a granodiorite upper continental crust composition. (Bottom) The dip in the Eu is also seen in the Nanxiong Basin and Buruanga clastic rocks but not from the Baguio data. Courtesy: R. A. B. Concepcion et al, International Journal of Earth Sciences 101, (2012).

________________

Journal Reference:

[1] Concepcion, R., Dimalanta, C., Yumul, G., Faustino-Eslava, D., Queaño, K., Tamayo, R., & Imai, A. (2012). Petrography, geochemistry, and tectonics of a rifted fragment of Mainland Asia: evidence from the Lasala Formation, Mindoro Island, Philippines International Journal of Earth Sciences, 101 (1), 273-290 DOI: 10.1007/s00531-011-0643-5

___________

Click an image to go to gallery view.

Advertisements