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The Cretaceous Southeast Bohol Ophiolite Complex, Central Philippines: a highly disaggregated supra-subduction zone ophiolite
Journal of Asian Earth Sciences 21 (2003) 957–965 www.elsevier.com/locate/jseaes
The Cretaceous Southeast Bohol Ophiolite Complex, Central Philippines: a highly disaggregated supra-subduction zone ophiolite Graciano P. Yumul Jr.* Rushurgent Working Group, National Institute of Geological Sciences, College of Science, University of the Philippines, Diliman, Quezon City 1101, Philippines Received 5 September 2000; revised 17 September 2001; accepted 31 May 2002
Abstract The Southeast Bohol Ophiolite Complex (SEBOC) represents a complete ophiolite sequence with a pelagic chert cap yielding an Early Cretaceous age. The residual peridotite suite of this ophiolite complex is dominated by harzburgite with rare occurrences of lherzolite and dunite pods. Layered ultramafic cumulate rocks, made up of clinopyroxenite, wehrlite, dunite, and massive to layered mafic cumulate rocks consisting of gabbro and norite, are present. Although disaggregated, the ophiolite complex includes a sheeted dike complex and a preponderance of sheet flows over pillow lavas in the volcanic section. Geologic observations such as gabbro pods included within residual harzburgite and deep-water sedimentary and volcanic rocks directly overlying the harzburgite have often been reported from the MidAtlantic Ridge and other slow-spreading centers. However, such situations are never encountered in the SEBOC. Geochemical data on SEBOC show that this ophiolite complex exhibits transitional Mid-Ocean Ridge basalt-island arc tholeiite geochemical affinities. This suggests formation in a subduction-related marginal basin. The field geological characteristics of SEBOC show that, aside from being a harzburgite ophiolite type, this crust – mantle sequence was formed in a fast-spreading center. Later tectonic events resulted in its emplacement as a forearc ophiolite. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Ophiolite; Fast-spreading center; Harzburgite ophiolite type; Bohol; Philippines
1. Introduction The wealth of information made available from the study of modern-day ocean basins has led to the realization that the physical configuration of the crust – mantle sequence, as exposed on the ocean floor, varies. To a first order, this is attributable to differences in the spreading rate of an ocean basin, which, to a certain extent, is related to the amount of magma extruded on the surface (Searle, 1992; Niu, 1997; Klingelhofer et al., 2000). Slow-spreading centers, which would include the Mid-Atlantic Ridge, Southwest Indian Ridge and the West Philippine Sea, are characterized by a full spreading rate of less than 5 cm/year and fluctuating magma supply with mechanical extension resulting primarily from faulting. Fast-spreading centers, with . 10 cm/year full spreading rates, are characterized by robust magmatism with sheet flows dominating over pillow lavas. Sediments intercalated within volcanic rocks indicate gaps * Tel.: þ63-2-436-8840; fax: þ 63-2-929-6047. E-mail address: [email protected] (G.P. Yumul).
in volcanism and are indicative of generation along intermediate spreading-rate centers (Karson, 1998; Dilek et al., 1998). As a consequence of these discoveries, our concept of the crust –mantle sequence has changed. The layered model from peridotite all the way to volcanic rocks, as exemplified by ophiolites following the definition of the Penrose Conference and viewed to represent the oceanic crust –mantle sequence, is still valid (Anonymous, 1972). These crust –mantle sequences are now recognized to have actually been generated along intermediate to fast-spreading centers. Crust – mantle sequences formed in slow-spreading centers expose thin crust as a result of limited magma supply. The mantle is emplaced almost at the surface due to fault-related extensions (Dick, 1989; Karson, 1998; Cannat, 1993). The Philippine island arc system is made up of ophiolite and ophiolitic complexes with varying geochemical affinities and structural configurations. The amalgamation, accretion and emplacement of these different crust –mantle sequences were brought about by a combination of processes that include subduction-related collision,
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onramping and strike-slip faulting among others (Yumul et al., 1997). In spite of the recognition that the evolution of the Philippine island arc system is punctuated by ocean basin closure and emplacement, there has never been any systematic attempt to characterize and to model the closed basins, through the study of ophiolites, in terms of their spreading rates. It is in this light that the evolution of the Southeast Bohol Ophiolite Complex (SEBOC) in the Central Philippines is discussed. It will be argued that the SEBOC was formed along a fast-spreading center and was emplaced within a forearc setting. The evolution of SEBOC is important in helping to elucidate the geologic history of the Central Philippines. This study may have important implications in our understanding and knowledge of how active margins evolved through time and space.
2. Geologic outline Bohol Island is part of the Central Philippines, which is bounded on the east by the Pliocene Philippine Trench and on the west by the Early to Middle Miocene Negros Trench (Fig. 1). The West Philippine Sea plate obliquely subducts along the Philippine Trench while the Sulu Sea goes down along the Negros Trench. Cutting through Leyte Island is the Pliocene Philippine Fault Zone, which accommodates whatever stress that cannot be taken up along the Philippine Trench. One noticeable aspect in the configuration of the islands in the Central Philippines is the difference in their trends. Panay, Negros, Cebu, Bohol and northwest Masbate (collectively called as the Western Visayan block) all trend NE –SW while Samar, Leyte and northeast Masbate all
show a NW – SE elongation. The NE – SW trend of the Western Visayan block is attributed to its clockwise rotation brought about by the collision of the Palawan microcontinental block with the Philippine mobile belt (McCabe et al., 1987; Yumul et al., 2000). Southeast Bohol, where the SEBOC is exposed, has for its basement the Cretaceous –Paleogene Alicia Schist. The Alicia Schist is made up of chlorite schist, quartz-mica schist and amphibolite, which resulted from regional metamorphism (Diegor et al., 1996; Faustino, 2000) (Figs. 2 and 3). The overlying formation, which is in thrust contact with the Alicia Schist, is the Cansiwang Melange. This tectonic melange is characterized by a serpentinite matrix that encloses various clasts (from hundreds of meters wide [megaclasts] to centimeters in width) that include harzburgite, basalt and chert. The melange is interpreted to have formed in a sediment-starved trench (De Jesus et al., 2000). The SEBOC is thrusted on top of the Cansiwang Melange. The observed relationship among the Alicia Schist, Cansiwang Melange and the SEBOC suggests emplacement of the ophiolite as a cold lithosphere on top of the Cansiwang Melange and Alicia Schist. This has been discussed and presented elsewhere (Barretto et al., 2000; De Jesus et al., 2000). The SEBOC is a complete, albeit, disaggregated ophiolite. Pillow lavas, sheet flows, a dike complex made up of basalt, diabase and microgabbro, layered to massive gabbro, thin clinopyroxenite, wehrlite, dunite layers and extensive harzburgite with rare lherzolite make up the SEBOC (Figs. 4 and 5(b) –(g)). The chert carapace of the SEBOC, based on radiolarian and foraminiferal assemblages, indicate an Early Cretaceous age (Faustino, 2000) (Fig. 5(a)). The ophiolite complex is unconformably
Fig. 1. Regional map of the Central Philippines showing the location of Bohol Island and the major tectonic features. The Sulu Sea plate subducts along the Negros Trench while the West Philippine Sea plate obliquely dives down along the Philippine Trench. The Philippine Fault is a left-lateral strike slip fault.
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Fig. 2. Geologic map of Southeast Bohol which shows the distribution of the SEBOC, Cansiwang Melange and the Alicia Schist. The left, upper inset is an enlarged geologic map of the Duero–Mayana area where the complete members of the SEBOC were mapped. The geologic map is a product of the 1997 University of the Philippines-National Institute of Geological Sciences (UP-NIGS) Geology 170 (Bohol Group) mapping campaign. The right, upper inset shows the location of the study area with respect to the NW Bohol arc. See text for discussion.
overlain by Miocene to Pleistocene, deep-marine turbidite clastic deposits (e.g. Carmen Formation) and shallow-water limestone (e.g. Sierra Bullones and Maribojoc Limestones) units. Magmatism is characterized by plug-like andesite intrusions (Jagna Andesite) and pyroclastic deposits (Lumbog Pyroclastics of the Carmen Formation) (Fig. 3).
3. The Southeast Bohol Ophiolite Complex 3.1. Residual harzburgite Northeast– southwest trending harzburgite exposures are found in Duero, Guindulman and Alicia (Fig. 2). Massive to highly fractured, fresh to intensely serpentinized harzburgite is encountered. Serpentinite shows bastite jutting out of the serpentinized matrix (hob-nail texture). The harzburgite is associated with a limited occurrence of lherzolite and
fine-grained, black to dark green dunite. The dunite is found within the harzburgite as lenses or pods. Based on field characteristics, the dunite and harzburgite are of residual origin. The rare lherzolite shows a protogranular texture and can only be differentiated from harzburgite under the microscope. The large, euhedral clinopyroxene present in the lherzolite is not consistent with the clinopyroxene forming by secondary impregnation. The harzburgite shows protogranular to porphyroclastic textures with holly leaf to euhedral chromian spinel. The dunite is equigranular with the chromian spinel ranging from euhedral to vermicular in shape. Serpentinization among some of the dunite is almost complete. Gabbro, anorthosite, epidosite and pyroxenite dikes cut the harzburgite (Fig. 5(g)). Although the dike margins manifest serpentinized host rocks, it is apparent that mantle – melt interaction had occurred since most of the aureoles are dunite which grade to harzburgite away from the dikes (Zhou et al., 1996; Quick and Gregory, 1995).
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3.2. Layered ultramafic cumulate rocks In Payao, Duero, an exposure of interlayered dunite, clinopyroxenite and wehrlite is encountered with a layering attitude of N658E strike and 708SE dip. The olive-green clinopyroxenite and wehrlite are coarse grained while the dunite is black and fine grained. The pyroxenites and dunite are serpentinized. A single chromite pod with a dunite aureole was also noted. Serpentinized layered ultramafic cumulate rocks were also observed along the coast in the Alicia area (ca. 98550 N lat.; 1248330 E long.) again made up of wehrlite, dunite and clinopyroxenite. Due to the disaggregation of the SEBOC, the contact between the residual harzburgite and the layered ultramafic cumulate rocks is not encountered. This is also true for the boundaries of all the rock units of the ophiolite complex. 3.3. Layered to massive gabbro
Fig. 3. General stratigraphy of Southeast Bohol as modified from the 1997 UP-NIGS Geology 170 results. Take note of the thrusting relationship of the SEBOC, Cansiwang Melange and the Alicia Schist.
Layered to massive gabbro made up of plagioclase and clinopyroxene is exposed in Alejawan and Payao, both in Duero (Fig. 5(f)). Some norite and hornblende-bearing gabbro are also encountered. The gabbro is generally medium- to coarse-grained and grayish-white to black in color depending on the ratio of feldspar with the mafic minerals. The gabbro, which is mostly adcumulate to mesocumulate, has a crystallization order of plagioclase ! clinopyroxene and plagioclase ! clinopyroxene ! hornblende whereas the norite indicates clinopyroxene ! orthopyroxene ! plagioclase. Although
Fig. 4. (a) Configuration of an ophiolite formed in a slow-spreading center corresponding to the LOT. Thickness from the volcanic sequence to the peridotite will be 2– 3 km. (b) Ophiolite configuration generated in a fast-spreading center which is similar to the HOT. Thickness from the volcanic sequence to the peridotite will be around 10 km. (c) Reconstructed ophiolite stratigraphy of the SEBOC. The thicknesses shown for the different members of the SEBOC are derived based on outcrops and contour expressions. See text for discussion.
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Fig. 5. (a) Pelagic chert, which was dated as Early Cretaceous, on top of a basalt flow as exposed along the Alejawan River. (b) Manganiferous umbers associated with pillow lavas along the Alejawan River. ((c) and (d)) Pillow lavas and sheet flows as encountered along the Alejawan River, respectively. (e) Sheeted dike complex in Payao, Duero. (f) Layered gabbro outcropping along the Alejawan River. (g) Harzburgite cut by anorthosite (white intrusive materials) as mapped in the Boctol–Mayana area. (h) Cansiwang Melange with basalt and harzburgite clasts in a serpentinized matrix as exposed in the Cansiwang –Labo road. Photographs (b), (c), (f), and (g) were taken by the Geology 170 Bohol Group.
exposed as patches, the massive gabbro and layered mafic cumulate rocks occur as distinct units that are not enveloped by peridotite. 3.4. Sheeted dike complex—sheet flows to pillow lavas The sheeted dike complex of SEBOC is exposed in the Bangwalog area in Duero (Fig. 5(e)). As exposed, a series of diabase, microgabbro, basalt and aplite dikes cut one another. The complex is made up of 100% dike and does
not have any country rock. The width of the dikes ranges from 10 to 20 cm, with the color varying from brown through green to yellowish green. Although the rocks have undergone low-grade greenschist-facies metamorphism as a result of ocean floor metamorphism, fresh plagioclase and clinopyroxene can still be identified. The dominant texture is intersertal to intergranular. Baking and chilling is present, although a preferential direction of chilling is absent. The dominant attitude of the dike complex is 408N–508E strike, 558 –658NW dip. The sheet flows and pillow lavas of the
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Fig. 6. Line drawing of an outcrop in the Cansiwang showing harzburgite thrusted over pillow lavas–sheet flows. All of these rocks are megaclasts (hundreds of meters wide) within the Cansiwang serpentinite melange.
SEBOC, capped by pelagic chert and manganiferous umbers, are dominantly exposed in Alejawan and Mayana. The lavas are massive and are not clasts or thrust packets within the Cansiwang Melange (Fig. 5(a) – (d)). Some of the lavas are altered to epidosite; almost all of the lavas have undergone greenschist-facies ocean floor metamorphism. Similar to what is noted in the dike complex, the pillow lavas and sheet flows have plagioclase and clinopyroxene. Textures range from intersertal through intergranular to spherulitic. An extensive outcrop of pillow lavas and sheet flows thrusted beneath the harzburgite is found in Cansiwang. The exposure shows a vertical repetition of the thrusting relationship between the volcanic rocks and the harzburgite. The whole outcrop is enveloped within the Cansiwang serpentinite melange, making the outcrop a megaclast (Figs. 5(h) and 6). The lava and sheet flows are overlain by chert, which is capped by tuffaceous sediments. Clinopyroxene, plagioclase and magnetite are found among the volcanic rocks; textures range from spherulitic to intersertal and intergranular.
plagioclase with composition An87Ab13 to An93Ab7 and pyroxene with low Mg# (Mg/Mg þ Fe2þ) within a range of 0.65 –0.85. Most of the harzburgite, lherzolite and pyroxenite have Cr# (Cr/Cr þ Al) 0.20 – 0.50 while dunite shows Cr# ranging from 0.40 to 0.80. The geochemical data for the volcanic, mafic – ultramafic cumulate and residual peridotite rocks, combined with the geological setting of the ophiolite as determined from the capping chert and limited tuffaceous sediments, suggest that the SEBOC was generated in a subduction-related marginal basin. The SEBOC is therefore a supra-subduction zone ophiolite. Reconstruction of the geologic events, as will be shown later, calls for the emplacement of SEBOC as a forearc ophiolite. This is consistent with the presence of boninitic rocks, the serpentinized-matrix Cansiwang Melange and the protoSoutheast Bohol Trench located offshore southeast (present geographic setting) of the SEBOC. These geological features are also observed in other forearc ophiolites (Hall, 1990; Ballantyne, 1992).
5. Discussion 4. Whole rock and mineral geochemistry 5.1. Evidence for generation in a fast-spreading center Available geochemical data show that the volcanic to hypabyssal rocks exhibit transitional Mid-Ocean Ridge basalt-island arc tholeiite signatures. Yumul et al. (1996) showed that most of the volcanic – hypabyssal rocks are tholeiitic. Boninitic, MORB-like, calc-alkaline basalt and andesitic rocks are also recognized (Faustino, 2000). The boninitic rocks (Zr/Y , 1; Ti/V , 20) overlying MORBlike (Zr=Y ¼ 3:5; Ti=V ¼ 30) volcanic rocks occur as pillow lavas and sheet flows which are megaclasts within the Cansiwang Melange (Fig. 6). The calc-alkaline basalt (Zr=Y ¼ 2:5 – 6; Ti=V ¼ 20 – 60) and andesite (Zr=Y ¼ 1:5 – 5; Ti=V ¼ 10 – 20) are dominantly found as in situ volcanic rocks. The gabbro and norite have
Slow-spreading centers contain tectonic features (e.g. megamullions) that are not readily apparent in intermediate to fast-spreading centers (Tucholke et al., 1998; Fujioka et al., 1999; Allerton et al., 2000). Submersible dives and dredging data, supported by geophysical information, show that slow-spreading centers are characterized by (a) small volume of gabbros and basalts, (b) general absence of dike complex, (c) deep-water sediments or volcanic rocks directly overlying peridotites, (d) large-scale extensional faulting, (e) discontinuous axial magma chambers, (f) localized hydrothermal deposits and (g) large-scale tilting of the crust (Cannat and Casey, 1995; Lagabrielle et al.,
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1998). As already mentioned, due to the paucity of magmatism and low degree of partial melting, the crust has time to solidify and strengthen since the main cause of mechanical extension is faulting (Ishiwatari, 1985; Karson, 1998). Volcanism is also characterized by point source volcanism with the lava flows dominated by pillow structures (Searle, 1992). With the robust magmatism in fast-spreading centers, volcanism is fissure-type with the dominance of sheet flows with respect to pillow lavas. The magmatic activities typical of fast-spreading centers ensure the existence of long-lived magma chambers (MacDonald, 1998). Comparing the SEBOC with the above geological attributes gives an idea of the kind of spreading center responsible for the generation of this ancient crust –mantle sequence. No deep-water sediment or pillow lava directly overlies the peridotite in the SEBOC. The layered and massive gabbro suggests the presence of a magma chamber (Figs. 4(c) and 5(f)). Slow-spreading centers have no welldeveloped magma chamber and are dominated by gabbro pods within the peridotite indicating episodic intrusion or a low magma budget (Fig. 4(a)). Aside from having a sheeted dike complex, basaltic sheet flows sometimes dominate over pillow lavas as encountered in Alejawan, Duero (Fig. 5(c) – (e)). Although the SEBOC is intensely faulted, this was not brought about by mechanical extension, as is the case for slow-spreading centers. The highly faulted nature of the SEBOC occurred after its generation and is attributed to emplacement as a forearc ophiolite. No sediment layer within the volcanic suite, which may indicate a gap in volcanism, was encountered. This negates the possibility of the SEBOC being formed in an intermediatespreading center. The Troodos Ophiolite has a well-developed sheeted dike complex, but is believed to have formed in a slowspreading center (Dilek et al., 1998). In spite of the presence of a sheeted dike complex, there is the possibility that the SEBOC was formed in the similar environment as that of the Troodos Ophiolite. However, no listric faults are observed to cut the dike complex that may suggest amagmatic extension. This feature was observed in the Troodos Ophiolite. Moreover, the absence of harzburgite capped by volcanic rocks or deep-sea sediments and the presence of massive to layered gabbro indicating a well-formed magma chamber do not support formation of the SEBOC in a slowspreading center. Although the rate of spreading may change through time, or even along the ridge axis (Lagabrielle and Lemoine, 1997), there is no compelling evidence to conclude that the SEBOC formed in a slowspreading environment. Ridge-transform intersections (RTI) of fast-spreading centers, similar to those observed in the Garrett and Terevaka transform faults, are noted to simulate the physical and magmatic characteristics of slow-spreading centers (Constantin, 1999). Such a feature was so far not observed in the SEBOC. Moreover, Fe– Ti basalts which characterize
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slow-spreading centers, RTI and propagating tips are not present in the SEBOC. In conclusion, on the basis of the observed field characteristics of the SEBOC, it is believed that this ophiolite complex was formed in a fastspreading center. 5.2. Harzburgite ophiolite type or lherzolite ophiolite type Crust – mantle sequences from slow-spreading centers correspond to the lherzolite ophiolite type (LOT) while those from fast-spreading centers are characterized by the harzburgite ophiolite type (HOT) (Nicolas and Al Azri, 1991). As presented in the works of Boudier and Nicolas (1985/86), the HOT is characterized by (a) a metamorphic sole made up of metamorphosed oceanic crust, (b) a thick cumulate gabbro layer, (c) an intrusive complex dominated by dikes, (d) volcanic rocks having tholeiitic characteristics and the mantle section composed of harzburgite and dunite, and (e) presence of chromite pods (Fig. 4(b)). The LOT is defined as having (a) a metamorphosed continental crust or oceanic crust as the metamorphic sole, (b) thin to poorly developed mafic cumulate layer, (c) the preponderance of sills over dikes, (d) the presence of tholeiitic to alkaline volcanic rocks with the mantle section characterized by plagioclase lherzolite and (e) rare to absent chromite pods (Fig. 4(a)). Looking at these varying parameters, it can readily be seen that the HOT has undergone greater degrees of cumulative partial melting as compared to LOT (Ishiwatari, 1991). This is in consonance with the dissimilarity between fast- and slow-spreading centers. Fast-spreading centers, characterized by robust magmatism, have mantle source regions that have undergone more stages and greater degrees of partial melting resulting in the formation of harzburgite as the residual peridotite (Niu and Hekinian, 1997). On the other hand, slow-spreading centers are characterized by episodic magmatism, which translates into lower degrees of partial melting relative to fast-spreading centers (Ishiwatari, 1985). As a result, clinopyroxene-bearing harzburgite or lherzolite, in general, characterizes the mantle peridotite section of slowspreading centers. The field characteristics of the SEBOC classifies it as a HOT. It contains tholeiitic pillow lavas and sheet flows with an associated hypabyssal dike complex and a mafic cumulate sequence. No alkaline rock is present. Nonetheless, instead of an amphibolite sole, the SEBOC is characterized by a low temperature metamorphic sole, the serpentinized-matrix Cansiwang Melange. Furthermore, there is a paucity of chromite mineralization in the SEBOC contrary to other HOT-classified crust –mantle sequences. It appears that the appropriate condition for the formation of chromite mineralization (e.g. high pH2O; high fO2; extensive mantle –melt interaction) was not achieved. Although there are few occurrences of lherzolite, no plagioclase lherzolite (which usually defines the LOT) is noted in the SEBOC.
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Lastly, the presence of the capping chert on the SEBOC suggests a relatively deep-water marginal basin which is at least below the carbonate compensation depth. The presence of a nearby landmass is suggested by the tuff overlying the chert. The SEBOC initially formed in a fast-spreading center (Fig. 7(a)). A change in the stress field, from an extensional to a compressional regime due to a local or regional tectonic event, could have resulted in the subduction of a slab beneath another crust – mantle sequence (Fig. 7(b)). The onramped oceanic crust –mantle sequence, which corresponds to the SEBOC, was possibly metasomatised by fluids coming from the subducted slab and may explain the observed island arc geochemical signature. The subduction also resulted in the formation of a volcanic arc which may correspond to the arc intrusives found in NW Bohol (Fig. 7(c)), thus classifying the SEBOC as a forearc ophiolite. The emplacement of the SEBOC followed by uplift and erosion, supplied materials to the Cansiwang Melange (Fig. 7(d)). This was followed by the collision of the Alicia Schist, which has been previously identified as an
oceanic bathymetric high of continent-derivation (Diegor et al., 1996; De Jesus et al., 2000).
6. Conclusions The SEBOC is a complete ophiolite sequence that was disaggregated due to syn- and post-emplacement faulting. The presence of a sheet flow and pillow lava unit, sheeted dike complex, massive to layered mafic –ultramafic cumulate rocks and harzburgite as the dominant residual peridotite suggest that the SEBOC was formed in a fastspreading center. The SEBOC, a supra-subduction zone ophiolite, is also classified as a HOT although no extensive chromite body is present and the metamorphic sole is not amphibolite, but rather serpentinite. The available data show that the SEBOC was initially generated in a subduction-related marginal basin. A portion of the marginal basin was onramped above a subducting slab. This resulted in emplacement of the crust –mantle sequence, corresponding to the SEBOC, as a forearc ophiolite.
Acknowledgments Field support extended by Mr W. Diegor and his colleagues at the Mines and Geosciences Bureau-Cebu Office is appreciated. The mapping of the ophiolite and its immediate vicinity would have not been possible without the support of the UP-NIGS RWG people and the UP-NIGS Geology 170 students (Bohol Group). Financial and logistic support came from the Department of Science and Technology, UP-National Institute of Geological Sciences, Japan Society for the Promotion of Science and Plantation Mining Inc. I would like to thank K. Tamaki, T. Ishii and A. Taira for the opportunity to work in the Ocean Research Institute, University of Tokyo where most of the ideas expressed here were developed. Discussions with K. Ozawa, H. Nagahara-Takahashi, H. Iwamori, R.A. Tamayo, Jr., C.B. Dimalanta, H. Sato and H. Taniguchi are appreciated. J.V. de Jesus and F.O. Olaguera provided support in making the figures. Constructive comments made by A. Ishiwatari and H. Maekawa on an early version significantly improved the paper. Lastly, I would like to extend my appreciation to Prof. H. Hirano for the invitation to participate in the IGCP 434 project.
Fig. 7. Tectonic evolution of the SEBOC. At (a), the ophiolite was generated in a fast-spreading center. This was followed by the initiation of subduction due to a change in the stress field from an extensional to a compressional one (b). The subduction event can account for the transitional MORB-IAT signatures of the SEBOC volcanic rocks and the formation of the volcanic arc in NW Bohol (c). This was followed by the formation of the Cansiwang Melange and the subsequent onramping of the ophiolite on top of the melange due to the collision of an oceanic bathymetric high, the Alicia Schist (d). See text for discussion.
References Allerton, S., Escartin, J., Searle, R.C., 2000. Extremely asymmetric magmatic accretion of oceanic crust at the ends of slow-spreading ridge segments. Geology 28, 179 –182. Anonymous, 1972. Penrose field conference on ophiolites. Geotimes 17, 24 –25. Ballantyne, P., 1992. Petrology and geochemistry of the plutonic rocks of the Halmahera Ophiolite, Eastern Indonesia: an analogue of modern
G.P. Yumul Jr. / Journal of Asian Earth Sciences 21 (2003) 957–965 oceanic forearcs. In: Parson, L.M., Murton, B.J., Browning, P. (Eds.), Ophiolites and their Modern Oceanic Analogues. Geological Society of London Special Publication 60, 179– 202. Barretto, J.L., Dimalanta, C.B., Yumul, G.P. Jr., 2000. Gravity variations along the Southeast Bohol Ophiolite Complex (SEBOC), Central Philippines: implications on ophiolite emplacement. The Island Arc 9, 576–584. Boudier, F., Nicolas, A., 1985/86. Harzburgite and lherzolite subtypes in ophiolitic oceanic environments. Earth & Planetary Science Letters 76, 84–92. Cannat, M., 1993. Emplacement of mantle rocks in the seafloor at MidOcean Ridges. Journal of Geophysical Research 98, 4163–4172. Cannat, M., Casey, J.F., 1995. An ultramafic lift at the Mid-Atlantic Ridge: successive stages of magmatism in serpentinized peridotites from the 158N Region. In: Vissers, R.L.M., Nicolas, A. (Eds.), Mantle and Lower Crust Exposed in Oceanic Ridges and in Ophiolites, Kluwer Academic Publishers, Dordrecht, pp. 5– 34. Constantin, M., 1999. Gabbroic intrusions and magmatic metasomatism in harzburgites from the Garrett Transform fault: implications for the nature of the mantle – crust transition at fast-spreading ridges. Contributions to Mineralogy and Petrology 136, 111–130. De Jesus, J.V., Yumul, G.P. Jr., Faustino, D.V., 2000. The Cansiwang Melange of Southeast Bohol (Central Philippines): origin and tectonic implications. The Island Arc 9, 566–575. Dick, H.J.B., 1989. Abyssal peridotites, very slow spreading ridges and ocean ridge magmatism. In: Saunders, A.D., Norris, M.J. (Eds.), Magmatism in the Ocean Basins. Geological Society of London Special Publication 42, 71 –105. Diegor, W.G., Aleta, D.A., Berado, A.E.G., Lucero, A.R. Jr., Miel, J.Z., Aleta, J.T., 1996. Field evidence of the Southeast Bohol Ophiolite Complex. In: Roger Buck, W., Delaney, P.T., Karson, J.A., Lagabrielle, Y. (Eds.), Faulting and Magmatism at Mid-Ocean Ridges. Journal of the Geological Society of the Philippines 51, 61– 72. Dilek, Y., Moores, E.M., Furnes, H., 1998. Structure of modern oceanic crust and ophiolites and implications for faulting and magmatism at oceanic spreading centers. Geophysical Monograph 106, 219 –265. Faustino, D.V., 2000. Geochemical and petrological characterization of the Southeast Bohol Ophiolite Complex (SEBOC): implications on the tectonic evolution of Southeast Bohol. Unpublished MSc Thesis, University of the Philippines. Fujioka, K., Okino, K., Kanamatsu, T., Ohara, Y., Ishizuka, O., Haraguchi, S., Ishii, T., 1999. Enigmatic extinct spreading center in the West Philippine backarc basin unveiled. Geology 27, 1135– 1138. Hall, R., 1990. Subduction-related ophiolite terrains: evidence from Southeast Asia. In: Malpas, J., Moores, E., Panayiotou, A., Xenophontos, C. (Eds.), Oceanic Crustal Analogues, Troodoos 1987, Geological Survey Department, Republic of Cyprus, pp. 449 –460. Ishiwatari, A., 1985. Alpine ophiolites: product of low-degree mantle melting in a Mesozoic transcurrent rift zone. Earth & Planetary Science Letters 76, 93–108. Ishiwatari, A., 1991. Time-space distribution and petrologic diversity of Japanese ophiolites. In: Peters, Tj., Nicolas, A., Coleman, R.G. (Eds.), Ophiolite Genesis and Evolution of the Oceanic Lithosphere, Kluwer Academic Publishers, Dordrecht, pp. 723 –743. Karson, J.A., 1998. Internal structure of oceanic lithosphere: a perspective from tectonic windows. In: Roger Buck, W., Delaney, P.T., Karson,
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J.A., Lagabrielle, Y. (Eds.), Faulting and Magmatism at Mid-Ocean Ridges. Geophysical Monograph 106, 177–218. Klingelhofer, F., Geli, L., White, R.S., 2000. Geophysical and geochemical constraints on crustal accretion at the very-slow spreading Mohns Ridge. Geophysical Research Letters 27, 1547–1550. Lagabrielle, Y., Lemoine, M., 1997. Alpine, Corsican and Apennine ophiolites: the slow-spreading ridge model. Comptes rendus de l’Academie des Sciences Paris, Sciences de la terre et des planets 325, 909–920. Lagabrielle, Y., Bideau, D., Cannat, M., Karson, J.A., Mevel, C., 1998. Ultramafic–mafic plutonic rock suites exposed along the Mid-Atlantic Ridge (108N–308N). In: Roger Buck, W., Delaney, P.T., Karson, J.A., Lagabrielle, Y. (Eds.), Faulting and Magmatism at Mid-Ocean Ridges. Symmetrical–asymmetrical distribution and implications for seafloor spreading processes. Geophysical Monograph 106, 153 –176. MacDonald, K.C., 1998. Linkages between faulting, volcanism, hydrothermal activity and segmentation on fast spreading centers. In: Roger Buck, W., Delaney, P.T., Karson, J.A., Lagabrielle, Y. (Eds.), Faulting and Magmatism at Mid-Ocean Ridges. Geophysical Monograph 106, 27– 58. McCabe, R., Kikawa, E., Cole, J., Malicse, A., Baldauf, P., Yumul, J., Almasco, J., 1987. Paleomagnetic results from Luzon and the Central Philippines. Journal of Geophysical Research 92, 555–580. Nicolas, A., Al Azri, H., 1991. Chromite-rich and chromite-poor ophiolites: the Oman Case. In: Peters, Tj., Nicolas, A., Coleman, R.G. (Eds.), Ophiolite Genesis and Evolution of the Oceanic Lithosphere, Kluwer Academic Publishers, Dordrecht, pp. 261–274. Niu, Y., 1997. Mantle melting and melt extraction processes beneath ocean ridges: evidence from abyssal peridotites. Journal of Petrology 38, 1047–1074. Niu, Y., Hekinian, R., 1997. Basaltic liquids and harzburgitic residues in the Garrett Transform: a case study of fast-spreading ridges. Earth & Planetary Science Letters 146, 243– 258. Quick, J.E., Gregory, R.T., 1995. Significance of melt– wall rock reaction: comparative anatomy of three ophiolites. Journal of Geology 103, 187– 198. Searle, R.C., 1992. The volcano-tectonic setting of oceanic lithosphere generation. In: Parson, L.M., Murton, B.J., Browning, P. (Eds.), Ophiolites and their Modern Oceanic Analogues. Geological Society Special Publication 60, 65–80. Tucholke, B.E., Lin, J., Kleinrock, M.C., 1998. Megamullions and mullion structure defining oceanic metamorphic core complexes on the MidAtlantic Ridge. Journal of Geophysical Research 103, 9857–9866. Yumul, G.P. Jr., Dimalanta, C.B., Jumawan, F.T., Tamayo, R.A. Jr., David, C.P.C., Rafols, J.P., 1996. Geology of the Southeast Bohol Ophiolite Complex: Implications and Significance, Geological Society of the Philippines, GEOCON 95 Proceedings, pp. 92–103. Yumul, G.P. Jr., Balce, G.R., Dimalanta, C.B., Datuin, R.T., 1997. Distribution, geochemistry and mineralization potentials of Philippine ophiolite and ophiolitic sequences. Ofioliti 22, 47–56. Yumul, G.P. Jr., Dimalanta, C.B., Tamayo, R.A. Jr., Barretto, J.L., 2000. Contrasting morphological trends of islands in Central Philippines: speculation on their origin. The Island Arc 9, 628–638. Zhou, M.-F., Robinson, P.T., Malpas, J., Li, Z., 1996. Podiform chromitites in the Luobosa ophiolite (Southern Tibet): implications for melt–rock interaction and chromite segregation in the upper mantle. Journal of Petrology 37, 3–12.
The Cretaceous Southeast Bohol Ophiolite Complex, Central Philippines: a highly disaggregated supra-subduction zone ophiolite Graciano P. Yumul Jr.* Rushurgent Working Group, National Institute of Geological Sciences, College of Science, University of the Philippines, Diliman, Quezon City 1101, Philippines Received 5 September 2000; revised 17 September 2001; accepted 31 May 2002
Abstract The Southeast Bohol Ophiolite Complex (SEBOC) represents a complete ophiolite sequence with a pelagic chert cap yielding an Early Cretaceous age. The residual peridotite suite of this ophiolite complex is dominated by harzburgite with rare occurrences of lherzolite and dunite pods. Layered ultramafic cumulate rocks, made up of clinopyroxenite, wehrlite, dunite, and massive to layered mafic cumulate rocks consisting of gabbro and norite, are present. Although disaggregated, the ophiolite complex includes a sheeted dike complex and a preponderance of sheet flows over pillow lavas in the volcanic section. Geologic observations such as gabbro pods included within residual harzburgite and deep-water sedimentary and volcanic rocks directly overlying the harzburgite have often been reported from the MidAtlantic Ridge and other slow-spreading centers. However, such situations are never encountered in the SEBOC. Geochemical data on SEBOC show that this ophiolite complex exhibits transitional Mid-Ocean Ridge basalt-island arc tholeiite geochemical affinities. This suggests formation in a subduction-related marginal basin. The field geological characteristics of SEBOC show that, aside from being a harzburgite ophiolite type, this crust – mantle sequence was formed in a fast-spreading center. Later tectonic events resulted in its emplacement as a forearc ophiolite. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Ophiolite; Fast-spreading center; Harzburgite ophiolite type; Bohol; Philippines
1. Introduction The wealth of information made available from the study of modern-day ocean basins has led to the realization that the physical configuration of the crust – mantle sequence, as exposed on the ocean floor, varies. To a first order, this is attributable to differences in the spreading rate of an ocean basin, which, to a certain extent, is related to the amount of magma extruded on the surface (Searle, 1992; Niu, 1997; Klingelhofer et al., 2000). Slow-spreading centers, which would include the Mid-Atlantic Ridge, Southwest Indian Ridge and the West Philippine Sea, are characterized by a full spreading rate of less than 5 cm/year and fluctuating magma supply with mechanical extension resulting primarily from faulting. Fast-spreading centers, with . 10 cm/year full spreading rates, are characterized by robust magmatism with sheet flows dominating over pillow lavas. Sediments intercalated within volcanic rocks indicate gaps * Tel.: þ63-2-436-8840; fax: þ 63-2-929-6047. E-mail address: [email protected] (G.P. Yumul).
in volcanism and are indicative of generation along intermediate spreading-rate centers (Karson, 1998; Dilek et al., 1998). As a consequence of these discoveries, our concept of the crust –mantle sequence has changed. The layered model from peridotite all the way to volcanic rocks, as exemplified by ophiolites following the definition of the Penrose Conference and viewed to represent the oceanic crust –mantle sequence, is still valid (Anonymous, 1972). These crust –mantle sequences are now recognized to have actually been generated along intermediate to fast-spreading centers. Crust – mantle sequences formed in slow-spreading centers expose thin crust as a result of limited magma supply. The mantle is emplaced almost at the surface due to fault-related extensions (Dick, 1989; Karson, 1998; Cannat, 1993). The Philippine island arc system is made up of ophiolite and ophiolitic complexes with varying geochemical affinities and structural configurations. The amalgamation, accretion and emplacement of these different crust –mantle sequences were brought about by a combination of processes that include subduction-related collision,
1367-9120/03/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 1 3 6 7 - 9 1 2 0 ( 0 2 ) 0 0 0 8 6 - X
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onramping and strike-slip faulting among others (Yumul et al., 1997). In spite of the recognition that the evolution of the Philippine island arc system is punctuated by ocean basin closure and emplacement, there has never been any systematic attempt to characterize and to model the closed basins, through the study of ophiolites, in terms of their spreading rates. It is in this light that the evolution of the Southeast Bohol Ophiolite Complex (SEBOC) in the Central Philippines is discussed. It will be argued that the SEBOC was formed along a fast-spreading center and was emplaced within a forearc setting. The evolution of SEBOC is important in helping to elucidate the geologic history of the Central Philippines. This study may have important implications in our understanding and knowledge of how active margins evolved through time and space.
2. Geologic outline Bohol Island is part of the Central Philippines, which is bounded on the east by the Pliocene Philippine Trench and on the west by the Early to Middle Miocene Negros Trench (Fig. 1). The West Philippine Sea plate obliquely subducts along the Philippine Trench while the Sulu Sea goes down along the Negros Trench. Cutting through Leyte Island is the Pliocene Philippine Fault Zone, which accommodates whatever stress that cannot be taken up along the Philippine Trench. One noticeable aspect in the configuration of the islands in the Central Philippines is the difference in their trends. Panay, Negros, Cebu, Bohol and northwest Masbate (collectively called as the Western Visayan block) all trend NE –SW while Samar, Leyte and northeast Masbate all
show a NW – SE elongation. The NE – SW trend of the Western Visayan block is attributed to its clockwise rotation brought about by the collision of the Palawan microcontinental block with the Philippine mobile belt (McCabe et al., 1987; Yumul et al., 2000). Southeast Bohol, where the SEBOC is exposed, has for its basement the Cretaceous –Paleogene Alicia Schist. The Alicia Schist is made up of chlorite schist, quartz-mica schist and amphibolite, which resulted from regional metamorphism (Diegor et al., 1996; Faustino, 2000) (Figs. 2 and 3). The overlying formation, which is in thrust contact with the Alicia Schist, is the Cansiwang Melange. This tectonic melange is characterized by a serpentinite matrix that encloses various clasts (from hundreds of meters wide [megaclasts] to centimeters in width) that include harzburgite, basalt and chert. The melange is interpreted to have formed in a sediment-starved trench (De Jesus et al., 2000). The SEBOC is thrusted on top of the Cansiwang Melange. The observed relationship among the Alicia Schist, Cansiwang Melange and the SEBOC suggests emplacement of the ophiolite as a cold lithosphere on top of the Cansiwang Melange and Alicia Schist. This has been discussed and presented elsewhere (Barretto et al., 2000; De Jesus et al., 2000). The SEBOC is a complete, albeit, disaggregated ophiolite. Pillow lavas, sheet flows, a dike complex made up of basalt, diabase and microgabbro, layered to massive gabbro, thin clinopyroxenite, wehrlite, dunite layers and extensive harzburgite with rare lherzolite make up the SEBOC (Figs. 4 and 5(b) –(g)). The chert carapace of the SEBOC, based on radiolarian and foraminiferal assemblages, indicate an Early Cretaceous age (Faustino, 2000) (Fig. 5(a)). The ophiolite complex is unconformably
Fig. 1. Regional map of the Central Philippines showing the location of Bohol Island and the major tectonic features. The Sulu Sea plate subducts along the Negros Trench while the West Philippine Sea plate obliquely dives down along the Philippine Trench. The Philippine Fault is a left-lateral strike slip fault.
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Fig. 2. Geologic map of Southeast Bohol which shows the distribution of the SEBOC, Cansiwang Melange and the Alicia Schist. The left, upper inset is an enlarged geologic map of the Duero–Mayana area where the complete members of the SEBOC were mapped. The geologic map is a product of the 1997 University of the Philippines-National Institute of Geological Sciences (UP-NIGS) Geology 170 (Bohol Group) mapping campaign. The right, upper inset shows the location of the study area with respect to the NW Bohol arc. See text for discussion.
overlain by Miocene to Pleistocene, deep-marine turbidite clastic deposits (e.g. Carmen Formation) and shallow-water limestone (e.g. Sierra Bullones and Maribojoc Limestones) units. Magmatism is characterized by plug-like andesite intrusions (Jagna Andesite) and pyroclastic deposits (Lumbog Pyroclastics of the Carmen Formation) (Fig. 3).
3. The Southeast Bohol Ophiolite Complex 3.1. Residual harzburgite Northeast– southwest trending harzburgite exposures are found in Duero, Guindulman and Alicia (Fig. 2). Massive to highly fractured, fresh to intensely serpentinized harzburgite is encountered. Serpentinite shows bastite jutting out of the serpentinized matrix (hob-nail texture). The harzburgite is associated with a limited occurrence of lherzolite and
fine-grained, black to dark green dunite. The dunite is found within the harzburgite as lenses or pods. Based on field characteristics, the dunite and harzburgite are of residual origin. The rare lherzolite shows a protogranular texture and can only be differentiated from harzburgite under the microscope. The large, euhedral clinopyroxene present in the lherzolite is not consistent with the clinopyroxene forming by secondary impregnation. The harzburgite shows protogranular to porphyroclastic textures with holly leaf to euhedral chromian spinel. The dunite is equigranular with the chromian spinel ranging from euhedral to vermicular in shape. Serpentinization among some of the dunite is almost complete. Gabbro, anorthosite, epidosite and pyroxenite dikes cut the harzburgite (Fig. 5(g)). Although the dike margins manifest serpentinized host rocks, it is apparent that mantle – melt interaction had occurred since most of the aureoles are dunite which grade to harzburgite away from the dikes (Zhou et al., 1996; Quick and Gregory, 1995).
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3.2. Layered ultramafic cumulate rocks In Payao, Duero, an exposure of interlayered dunite, clinopyroxenite and wehrlite is encountered with a layering attitude of N658E strike and 708SE dip. The olive-green clinopyroxenite and wehrlite are coarse grained while the dunite is black and fine grained. The pyroxenites and dunite are serpentinized. A single chromite pod with a dunite aureole was also noted. Serpentinized layered ultramafic cumulate rocks were also observed along the coast in the Alicia area (ca. 98550 N lat.; 1248330 E long.) again made up of wehrlite, dunite and clinopyroxenite. Due to the disaggregation of the SEBOC, the contact between the residual harzburgite and the layered ultramafic cumulate rocks is not encountered. This is also true for the boundaries of all the rock units of the ophiolite complex. 3.3. Layered to massive gabbro
Fig. 3. General stratigraphy of Southeast Bohol as modified from the 1997 UP-NIGS Geology 170 results. Take note of the thrusting relationship of the SEBOC, Cansiwang Melange and the Alicia Schist.
Layered to massive gabbro made up of plagioclase and clinopyroxene is exposed in Alejawan and Payao, both in Duero (Fig. 5(f)). Some norite and hornblende-bearing gabbro are also encountered. The gabbro is generally medium- to coarse-grained and grayish-white to black in color depending on the ratio of feldspar with the mafic minerals. The gabbro, which is mostly adcumulate to mesocumulate, has a crystallization order of plagioclase ! clinopyroxene and plagioclase ! clinopyroxene ! hornblende whereas the norite indicates clinopyroxene ! orthopyroxene ! plagioclase. Although
Fig. 4. (a) Configuration of an ophiolite formed in a slow-spreading center corresponding to the LOT. Thickness from the volcanic sequence to the peridotite will be 2– 3 km. (b) Ophiolite configuration generated in a fast-spreading center which is similar to the HOT. Thickness from the volcanic sequence to the peridotite will be around 10 km. (c) Reconstructed ophiolite stratigraphy of the SEBOC. The thicknesses shown for the different members of the SEBOC are derived based on outcrops and contour expressions. See text for discussion.
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Fig. 5. (a) Pelagic chert, which was dated as Early Cretaceous, on top of a basalt flow as exposed along the Alejawan River. (b) Manganiferous umbers associated with pillow lavas along the Alejawan River. ((c) and (d)) Pillow lavas and sheet flows as encountered along the Alejawan River, respectively. (e) Sheeted dike complex in Payao, Duero. (f) Layered gabbro outcropping along the Alejawan River. (g) Harzburgite cut by anorthosite (white intrusive materials) as mapped in the Boctol–Mayana area. (h) Cansiwang Melange with basalt and harzburgite clasts in a serpentinized matrix as exposed in the Cansiwang –Labo road. Photographs (b), (c), (f), and (g) were taken by the Geology 170 Bohol Group.
exposed as patches, the massive gabbro and layered mafic cumulate rocks occur as distinct units that are not enveloped by peridotite. 3.4. Sheeted dike complex—sheet flows to pillow lavas The sheeted dike complex of SEBOC is exposed in the Bangwalog area in Duero (Fig. 5(e)). As exposed, a series of diabase, microgabbro, basalt and aplite dikes cut one another. The complex is made up of 100% dike and does
not have any country rock. The width of the dikes ranges from 10 to 20 cm, with the color varying from brown through green to yellowish green. Although the rocks have undergone low-grade greenschist-facies metamorphism as a result of ocean floor metamorphism, fresh plagioclase and clinopyroxene can still be identified. The dominant texture is intersertal to intergranular. Baking and chilling is present, although a preferential direction of chilling is absent. The dominant attitude of the dike complex is 408N–508E strike, 558 –658NW dip. The sheet flows and pillow lavas of the
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Fig. 6. Line drawing of an outcrop in the Cansiwang showing harzburgite thrusted over pillow lavas–sheet flows. All of these rocks are megaclasts (hundreds of meters wide) within the Cansiwang serpentinite melange.
SEBOC, capped by pelagic chert and manganiferous umbers, are dominantly exposed in Alejawan and Mayana. The lavas are massive and are not clasts or thrust packets within the Cansiwang Melange (Fig. 5(a) – (d)). Some of the lavas are altered to epidosite; almost all of the lavas have undergone greenschist-facies ocean floor metamorphism. Similar to what is noted in the dike complex, the pillow lavas and sheet flows have plagioclase and clinopyroxene. Textures range from intersertal through intergranular to spherulitic. An extensive outcrop of pillow lavas and sheet flows thrusted beneath the harzburgite is found in Cansiwang. The exposure shows a vertical repetition of the thrusting relationship between the volcanic rocks and the harzburgite. The whole outcrop is enveloped within the Cansiwang serpentinite melange, making the outcrop a megaclast (Figs. 5(h) and 6). The lava and sheet flows are overlain by chert, which is capped by tuffaceous sediments. Clinopyroxene, plagioclase and magnetite are found among the volcanic rocks; textures range from spherulitic to intersertal and intergranular.
plagioclase with composition An87Ab13 to An93Ab7 and pyroxene with low Mg# (Mg/Mg þ Fe2þ) within a range of 0.65 –0.85. Most of the harzburgite, lherzolite and pyroxenite have Cr# (Cr/Cr þ Al) 0.20 – 0.50 while dunite shows Cr# ranging from 0.40 to 0.80. The geochemical data for the volcanic, mafic – ultramafic cumulate and residual peridotite rocks, combined with the geological setting of the ophiolite as determined from the capping chert and limited tuffaceous sediments, suggest that the SEBOC was generated in a subduction-related marginal basin. The SEBOC is therefore a supra-subduction zone ophiolite. Reconstruction of the geologic events, as will be shown later, calls for the emplacement of SEBOC as a forearc ophiolite. This is consistent with the presence of boninitic rocks, the serpentinized-matrix Cansiwang Melange and the protoSoutheast Bohol Trench located offshore southeast (present geographic setting) of the SEBOC. These geological features are also observed in other forearc ophiolites (Hall, 1990; Ballantyne, 1992).
5. Discussion 4. Whole rock and mineral geochemistry 5.1. Evidence for generation in a fast-spreading center Available geochemical data show that the volcanic to hypabyssal rocks exhibit transitional Mid-Ocean Ridge basalt-island arc tholeiite signatures. Yumul et al. (1996) showed that most of the volcanic – hypabyssal rocks are tholeiitic. Boninitic, MORB-like, calc-alkaline basalt and andesitic rocks are also recognized (Faustino, 2000). The boninitic rocks (Zr/Y , 1; Ti/V , 20) overlying MORBlike (Zr=Y ¼ 3:5; Ti=V ¼ 30) volcanic rocks occur as pillow lavas and sheet flows which are megaclasts within the Cansiwang Melange (Fig. 6). The calc-alkaline basalt (Zr=Y ¼ 2:5 – 6; Ti=V ¼ 20 – 60) and andesite (Zr=Y ¼ 1:5 – 5; Ti=V ¼ 10 – 20) are dominantly found as in situ volcanic rocks. The gabbro and norite have
Slow-spreading centers contain tectonic features (e.g. megamullions) that are not readily apparent in intermediate to fast-spreading centers (Tucholke et al., 1998; Fujioka et al., 1999; Allerton et al., 2000). Submersible dives and dredging data, supported by geophysical information, show that slow-spreading centers are characterized by (a) small volume of gabbros and basalts, (b) general absence of dike complex, (c) deep-water sediments or volcanic rocks directly overlying peridotites, (d) large-scale extensional faulting, (e) discontinuous axial magma chambers, (f) localized hydrothermal deposits and (g) large-scale tilting of the crust (Cannat and Casey, 1995; Lagabrielle et al.,
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1998). As already mentioned, due to the paucity of magmatism and low degree of partial melting, the crust has time to solidify and strengthen since the main cause of mechanical extension is faulting (Ishiwatari, 1985; Karson, 1998). Volcanism is also characterized by point source volcanism with the lava flows dominated by pillow structures (Searle, 1992). With the robust magmatism in fast-spreading centers, volcanism is fissure-type with the dominance of sheet flows with respect to pillow lavas. The magmatic activities typical of fast-spreading centers ensure the existence of long-lived magma chambers (MacDonald, 1998). Comparing the SEBOC with the above geological attributes gives an idea of the kind of spreading center responsible for the generation of this ancient crust –mantle sequence. No deep-water sediment or pillow lava directly overlies the peridotite in the SEBOC. The layered and massive gabbro suggests the presence of a magma chamber (Figs. 4(c) and 5(f)). Slow-spreading centers have no welldeveloped magma chamber and are dominated by gabbro pods within the peridotite indicating episodic intrusion or a low magma budget (Fig. 4(a)). Aside from having a sheeted dike complex, basaltic sheet flows sometimes dominate over pillow lavas as encountered in Alejawan, Duero (Fig. 5(c) – (e)). Although the SEBOC is intensely faulted, this was not brought about by mechanical extension, as is the case for slow-spreading centers. The highly faulted nature of the SEBOC occurred after its generation and is attributed to emplacement as a forearc ophiolite. No sediment layer within the volcanic suite, which may indicate a gap in volcanism, was encountered. This negates the possibility of the SEBOC being formed in an intermediatespreading center. The Troodos Ophiolite has a well-developed sheeted dike complex, but is believed to have formed in a slowspreading center (Dilek et al., 1998). In spite of the presence of a sheeted dike complex, there is the possibility that the SEBOC was formed in the similar environment as that of the Troodos Ophiolite. However, no listric faults are observed to cut the dike complex that may suggest amagmatic extension. This feature was observed in the Troodos Ophiolite. Moreover, the absence of harzburgite capped by volcanic rocks or deep-sea sediments and the presence of massive to layered gabbro indicating a well-formed magma chamber do not support formation of the SEBOC in a slowspreading center. Although the rate of spreading may change through time, or even along the ridge axis (Lagabrielle and Lemoine, 1997), there is no compelling evidence to conclude that the SEBOC formed in a slowspreading environment. Ridge-transform intersections (RTI) of fast-spreading centers, similar to those observed in the Garrett and Terevaka transform faults, are noted to simulate the physical and magmatic characteristics of slow-spreading centers (Constantin, 1999). Such a feature was so far not observed in the SEBOC. Moreover, Fe– Ti basalts which characterize
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slow-spreading centers, RTI and propagating tips are not present in the SEBOC. In conclusion, on the basis of the observed field characteristics of the SEBOC, it is believed that this ophiolite complex was formed in a fastspreading center. 5.2. Harzburgite ophiolite type or lherzolite ophiolite type Crust – mantle sequences from slow-spreading centers correspond to the lherzolite ophiolite type (LOT) while those from fast-spreading centers are characterized by the harzburgite ophiolite type (HOT) (Nicolas and Al Azri, 1991). As presented in the works of Boudier and Nicolas (1985/86), the HOT is characterized by (a) a metamorphic sole made up of metamorphosed oceanic crust, (b) a thick cumulate gabbro layer, (c) an intrusive complex dominated by dikes, (d) volcanic rocks having tholeiitic characteristics and the mantle section composed of harzburgite and dunite, and (e) presence of chromite pods (Fig. 4(b)). The LOT is defined as having (a) a metamorphosed continental crust or oceanic crust as the metamorphic sole, (b) thin to poorly developed mafic cumulate layer, (c) the preponderance of sills over dikes, (d) the presence of tholeiitic to alkaline volcanic rocks with the mantle section characterized by plagioclase lherzolite and (e) rare to absent chromite pods (Fig. 4(a)). Looking at these varying parameters, it can readily be seen that the HOT has undergone greater degrees of cumulative partial melting as compared to LOT (Ishiwatari, 1991). This is in consonance with the dissimilarity between fast- and slow-spreading centers. Fast-spreading centers, characterized by robust magmatism, have mantle source regions that have undergone more stages and greater degrees of partial melting resulting in the formation of harzburgite as the residual peridotite (Niu and Hekinian, 1997). On the other hand, slow-spreading centers are characterized by episodic magmatism, which translates into lower degrees of partial melting relative to fast-spreading centers (Ishiwatari, 1985). As a result, clinopyroxene-bearing harzburgite or lherzolite, in general, characterizes the mantle peridotite section of slowspreading centers. The field characteristics of the SEBOC classifies it as a HOT. It contains tholeiitic pillow lavas and sheet flows with an associated hypabyssal dike complex and a mafic cumulate sequence. No alkaline rock is present. Nonetheless, instead of an amphibolite sole, the SEBOC is characterized by a low temperature metamorphic sole, the serpentinized-matrix Cansiwang Melange. Furthermore, there is a paucity of chromite mineralization in the SEBOC contrary to other HOT-classified crust –mantle sequences. It appears that the appropriate condition for the formation of chromite mineralization (e.g. high pH2O; high fO2; extensive mantle –melt interaction) was not achieved. Although there are few occurrences of lherzolite, no plagioclase lherzolite (which usually defines the LOT) is noted in the SEBOC.
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Lastly, the presence of the capping chert on the SEBOC suggests a relatively deep-water marginal basin which is at least below the carbonate compensation depth. The presence of a nearby landmass is suggested by the tuff overlying the chert. The SEBOC initially formed in a fast-spreading center (Fig. 7(a)). A change in the stress field, from an extensional to a compressional regime due to a local or regional tectonic event, could have resulted in the subduction of a slab beneath another crust – mantle sequence (Fig. 7(b)). The onramped oceanic crust –mantle sequence, which corresponds to the SEBOC, was possibly metasomatised by fluids coming from the subducted slab and may explain the observed island arc geochemical signature. The subduction also resulted in the formation of a volcanic arc which may correspond to the arc intrusives found in NW Bohol (Fig. 7(c)), thus classifying the SEBOC as a forearc ophiolite. The emplacement of the SEBOC followed by uplift and erosion, supplied materials to the Cansiwang Melange (Fig. 7(d)). This was followed by the collision of the Alicia Schist, which has been previously identified as an
oceanic bathymetric high of continent-derivation (Diegor et al., 1996; De Jesus et al., 2000).
6. Conclusions The SEBOC is a complete ophiolite sequence that was disaggregated due to syn- and post-emplacement faulting. The presence of a sheet flow and pillow lava unit, sheeted dike complex, massive to layered mafic –ultramafic cumulate rocks and harzburgite as the dominant residual peridotite suggest that the SEBOC was formed in a fastspreading center. The SEBOC, a supra-subduction zone ophiolite, is also classified as a HOT although no extensive chromite body is present and the metamorphic sole is not amphibolite, but rather serpentinite. The available data show that the SEBOC was initially generated in a subduction-related marginal basin. A portion of the marginal basin was onramped above a subducting slab. This resulted in emplacement of the crust –mantle sequence, corresponding to the SEBOC, as a forearc ophiolite.
Acknowledgments Field support extended by Mr W. Diegor and his colleagues at the Mines and Geosciences Bureau-Cebu Office is appreciated. The mapping of the ophiolite and its immediate vicinity would have not been possible without the support of the UP-NIGS RWG people and the UP-NIGS Geology 170 students (Bohol Group). Financial and logistic support came from the Department of Science and Technology, UP-National Institute of Geological Sciences, Japan Society for the Promotion of Science and Plantation Mining Inc. I would like to thank K. Tamaki, T. Ishii and A. Taira for the opportunity to work in the Ocean Research Institute, University of Tokyo where most of the ideas expressed here were developed. Discussions with K. Ozawa, H. Nagahara-Takahashi, H. Iwamori, R.A. Tamayo, Jr., C.B. Dimalanta, H. Sato and H. Taniguchi are appreciated. J.V. de Jesus and F.O. Olaguera provided support in making the figures. Constructive comments made by A. Ishiwatari and H. Maekawa on an early version significantly improved the paper. Lastly, I would like to extend my appreciation to Prof. H. Hirano for the invitation to participate in the IGCP 434 project.
Fig. 7. Tectonic evolution of the SEBOC. At (a), the ophiolite was generated in a fast-spreading center. This was followed by the initiation of subduction due to a change in the stress field from an extensional to a compressional one (b). The subduction event can account for the transitional MORB-IAT signatures of the SEBOC volcanic rocks and the formation of the volcanic arc in NW Bohol (c). This was followed by the formation of the Cansiwang Melange and the subsequent onramping of the ophiolite on top of the melange due to the collision of an oceanic bathymetric high, the Alicia Schist (d). See text for discussion.
References Allerton, S., Escartin, J., Searle, R.C., 2000. Extremely asymmetric magmatic accretion of oceanic crust at the ends of slow-spreading ridge segments. Geology 28, 179 –182. Anonymous, 1972. Penrose field conference on ophiolites. Geotimes 17, 24 –25. Ballantyne, P., 1992. Petrology and geochemistry of the plutonic rocks of the Halmahera Ophiolite, Eastern Indonesia: an analogue of modern
G.P. Yumul Jr. / Journal of Asian Earth Sciences 21 (2003) 957–965 oceanic forearcs. In: Parson, L.M., Murton, B.J., Browning, P. (Eds.), Ophiolites and their Modern Oceanic Analogues. Geological Society of London Special Publication 60, 179– 202. Barretto, J.L., Dimalanta, C.B., Yumul, G.P. Jr., 2000. Gravity variations along the Southeast Bohol Ophiolite Complex (SEBOC), Central Philippines: implications on ophiolite emplacement. The Island Arc 9, 576–584. Boudier, F., Nicolas, A., 1985/86. Harzburgite and lherzolite subtypes in ophiolitic oceanic environments. Earth & Planetary Science Letters 76, 84–92. Cannat, M., 1993. Emplacement of mantle rocks in the seafloor at MidOcean Ridges. Journal of Geophysical Research 98, 4163–4172. Cannat, M., Casey, J.F., 1995. An ultramafic lift at the Mid-Atlantic Ridge: successive stages of magmatism in serpentinized peridotites from the 158N Region. In: Vissers, R.L.M., Nicolas, A. (Eds.), Mantle and Lower Crust Exposed in Oceanic Ridges and in Ophiolites, Kluwer Academic Publishers, Dordrecht, pp. 5– 34. Constantin, M., 1999. Gabbroic intrusions and magmatic metasomatism in harzburgites from the Garrett Transform fault: implications for the nature of the mantle – crust transition at fast-spreading ridges. Contributions to Mineralogy and Petrology 136, 111–130. De Jesus, J.V., Yumul, G.P. Jr., Faustino, D.V., 2000. The Cansiwang Melange of Southeast Bohol (Central Philippines): origin and tectonic implications. The Island Arc 9, 566–575. Dick, H.J.B., 1989. Abyssal peridotites, very slow spreading ridges and ocean ridge magmatism. In: Saunders, A.D., Norris, M.J. (Eds.), Magmatism in the Ocean Basins. Geological Society of London Special Publication 42, 71 –105. Diegor, W.G., Aleta, D.A., Berado, A.E.G., Lucero, A.R. Jr., Miel, J.Z., Aleta, J.T., 1996. Field evidence of the Southeast Bohol Ophiolite Complex. In: Roger Buck, W., Delaney, P.T., Karson, J.A., Lagabrielle, Y. (Eds.), Faulting and Magmatism at Mid-Ocean Ridges. Journal of the Geological Society of the Philippines 51, 61– 72. Dilek, Y., Moores, E.M., Furnes, H., 1998. Structure of modern oceanic crust and ophiolites and implications for faulting and magmatism at oceanic spreading centers. Geophysical Monograph 106, 219 –265. Faustino, D.V., 2000. Geochemical and petrological characterization of the Southeast Bohol Ophiolite Complex (SEBOC): implications on the tectonic evolution of Southeast Bohol. Unpublished MSc Thesis, University of the Philippines. Fujioka, K., Okino, K., Kanamatsu, T., Ohara, Y., Ishizuka, O., Haraguchi, S., Ishii, T., 1999. Enigmatic extinct spreading center in the West Philippine backarc basin unveiled. Geology 27, 1135– 1138. Hall, R., 1990. Subduction-related ophiolite terrains: evidence from Southeast Asia. In: Malpas, J., Moores, E., Panayiotou, A., Xenophontos, C. (Eds.), Oceanic Crustal Analogues, Troodoos 1987, Geological Survey Department, Republic of Cyprus, pp. 449 –460. Ishiwatari, A., 1985. Alpine ophiolites: product of low-degree mantle melting in a Mesozoic transcurrent rift zone. Earth & Planetary Science Letters 76, 93–108. Ishiwatari, A., 1991. Time-space distribution and petrologic diversity of Japanese ophiolites. In: Peters, Tj., Nicolas, A., Coleman, R.G. (Eds.), Ophiolite Genesis and Evolution of the Oceanic Lithosphere, Kluwer Academic Publishers, Dordrecht, pp. 723 –743. Karson, J.A., 1998. Internal structure of oceanic lithosphere: a perspective from tectonic windows. In: Roger Buck, W., Delaney, P.T., Karson,
965
J.A., Lagabrielle, Y. (Eds.), Faulting and Magmatism at Mid-Ocean Ridges. Geophysical Monograph 106, 177–218. Klingelhofer, F., Geli, L., White, R.S., 2000. Geophysical and geochemical constraints on crustal accretion at the very-slow spreading Mohns Ridge. Geophysical Research Letters 27, 1547–1550. Lagabrielle, Y., Lemoine, M., 1997. Alpine, Corsican and Apennine ophiolites: the slow-spreading ridge model. Comptes rendus de l’Academie des Sciences Paris, Sciences de la terre et des planets 325, 909–920. Lagabrielle, Y., Bideau, D., Cannat, M., Karson, J.A., Mevel, C., 1998. Ultramafic–mafic plutonic rock suites exposed along the Mid-Atlantic Ridge (108N–308N). In: Roger Buck, W., Delaney, P.T., Karson, J.A., Lagabrielle, Y. (Eds.), Faulting and Magmatism at Mid-Ocean Ridges. Symmetrical–asymmetrical distribution and implications for seafloor spreading processes. Geophysical Monograph 106, 153 –176. MacDonald, K.C., 1998. Linkages between faulting, volcanism, hydrothermal activity and segmentation on fast spreading centers. In: Roger Buck, W., Delaney, P.T., Karson, J.A., Lagabrielle, Y. (Eds.), Faulting and Magmatism at Mid-Ocean Ridges. Geophysical Monograph 106, 27– 58. McCabe, R., Kikawa, E., Cole, J., Malicse, A., Baldauf, P., Yumul, J., Almasco, J., 1987. Paleomagnetic results from Luzon and the Central Philippines. Journal of Geophysical Research 92, 555–580. Nicolas, A., Al Azri, H., 1991. Chromite-rich and chromite-poor ophiolites: the Oman Case. In: Peters, Tj., Nicolas, A., Coleman, R.G. (Eds.), Ophiolite Genesis and Evolution of the Oceanic Lithosphere, Kluwer Academic Publishers, Dordrecht, pp. 261–274. Niu, Y., 1997. Mantle melting and melt extraction processes beneath ocean ridges: evidence from abyssal peridotites. Journal of Petrology 38, 1047–1074. Niu, Y., Hekinian, R., 1997. Basaltic liquids and harzburgitic residues in the Garrett Transform: a case study of fast-spreading ridges. Earth & Planetary Science Letters 146, 243– 258. Quick, J.E., Gregory, R.T., 1995. Significance of melt– wall rock reaction: comparative anatomy of three ophiolites. Journal of Geology 103, 187– 198. Searle, R.C., 1992. The volcano-tectonic setting of oceanic lithosphere generation. In: Parson, L.M., Murton, B.J., Browning, P. (Eds.), Ophiolites and their Modern Oceanic Analogues. Geological Society Special Publication 60, 65–80. Tucholke, B.E., Lin, J., Kleinrock, M.C., 1998. Megamullions and mullion structure defining oceanic metamorphic core complexes on the MidAtlantic Ridge. Journal of Geophysical Research 103, 9857–9866. Yumul, G.P. Jr., Dimalanta, C.B., Jumawan, F.T., Tamayo, R.A. Jr., David, C.P.C., Rafols, J.P., 1996. Geology of the Southeast Bohol Ophiolite Complex: Implications and Significance, Geological Society of the Philippines, GEOCON 95 Proceedings, pp. 92–103. Yumul, G.P. Jr., Balce, G.R., Dimalanta, C.B., Datuin, R.T., 1997. Distribution, geochemistry and mineralization potentials of Philippine ophiolite and ophiolitic sequences. Ofioliti 22, 47–56. Yumul, G.P. Jr., Dimalanta, C.B., Tamayo, R.A. Jr., Barretto, J.L., 2000. Contrasting morphological trends of islands in Central Philippines: speculation on their origin. The Island Arc 9, 628–638. Zhou, M.-F., Robinson, P.T., Malpas, J., Li, Z., 1996. Podiform chromitites in the Luobosa ophiolite (Southern Tibet): implications for melt–rock interaction and chromite segregation in the upper mantle. Journal of Petrology 37, 3–12.