Arc magmatism and mineralization in North Luzon and its relationship to subduction at the East Luzon and North Manila Trenches

Journal of Southeast Asian Earth Sciences, Vol. 2, No. 2, pp. 79-93, 1988 Printed in Great Britain

0743-9547/88 $3.00 + 0.00 Pergamon Press pie

Arc magmatism and mineralization in North Luzon and its relationship to subduction at the East Luzon and North Manila Trenches JOHN A. WOLFE Taysan Copper Inc., MCC P.O. Box 1868, Makati, Metro Manila, Philippines (Received 21 November 1986; revised version accepted 22 September 1987) Abstract--The Tertiary tectonics of North Luzon are complicated by an early thermotectonic regime in the Eocene (50--40 Ma) and the second from 30 to 17 Ma in the Oligocene resulting from subduction in the East Luzon Trench. The second stage coincided with the opening of the South China Sea on the west side of the Philippines. Portions of the western Philippines were translated south from China by the opening of the South China Sea. This includes Mindoro Island, Palawan and the Reed Bank area. No one has presented any evidence that any oceanic crust existed between early Luzon and China prior to opening of the South China Sea. After spreading of the South China Sea ceased, China began to extrude eastward and coupled with the oceanic crust of the South China Sea initiated subduction in the North Manila trench under Luzon at about 17 Ma. Commencing at approximately 15 Ma a graben formed east of the Manila Trench, centered in Baguio City. It contained the volcanic arc which began to develop as the Agno batholith intruded the graben. The graben extends for at least 75 km on the southwest flank of the Cordillera with relayed extensions into the Cordillera. Porphyry copper mineralization developed within the graben from 10 to 8 Ma interrupted by the explosion of a caldera or volcano tectonic depression extending south of Baguio. This graben contains 22 porphyry copper bodies, some of them uneconomic. Described by Gervasio as a "crackle zone", the same zone was described by Fernandez and Damasco as the area most favorable for gold exploration. The second period of mineralization was imposed on the district from 4 to 3 Ma. Gold mineralization in the Baguio district constituted a third phase of mineralization in the Pleistocene. Absence of commercial mineralization in the Cordillera and Sierra Madre correlated with the Paleogene is one of the criteria for distinguishing between the subduction related to the South China Sea and that related to the Philippine Sea on the east. One of the important conclusions of this paper is that the Oligocene phase of subduction on the East Luzon Trench coincided with the opening of the South China Sea and both of these activities ceased when spreading on the South China Sea ceased. The period of mineralization in the graben at Baguio relates to the Manila Trench on the west side of Luzon with igneous activity commencing about 15 Ma.

INTRODUCTION

TECTONIC SETTING

EXTENSIVE studies of the Luzon transect initiated under the IDOE program have greatly expanded knowledge of Luzon Island and the seas to both the east and west. This paper attempts to separate the effects of the tectonics of the extinct Paleogene East Luzon trench (Wolfe 1983) from the Miocene to Recent North Manila Trench on the western side of the archipelago and to relate the volcanic arc of the Manila Trench to the copper and gold mineralization in the important mining district at Baguio City, Philippines. The district is located in the Cordilleran region of north Luzon Island, 250 km north of Manila. It has produced over 300 metric tons of gold and is the largest producing district in Southeast Asia. It has produced 30% of the output of copper of the Philippines (Balce et al. 1980). Figure 1 is a general orientation map of the region.

The Philippine Archipelago extends for about 1500 km generally along longitude line 122° E, from 5 to 21 ° N. Figure 2 is a location map of the tectonic elements of North Luzon showing the relationship among the Philippines, the South China Sea, the East Luzon and Manila Trenches, the Luzon Fault and the Philippine Sea. Throughout the Jurassic and Cretaceous the Pacific plate moved northward. At about 43 Ma B.P. there were major realignments in the Pacific region. As shown by the change in direction of the Emperor seamounts west of Hawaii, the Pacific plate began moving west northwest instead of north (Morgan 1972). This is approximately the time when India impacted south Asia (Tapponnier et al. 1982). A hiatus in plutonism in north Luzon is suggested by lack of dates on plutons from 43 to 33 Ma corresponding to the same gap in volcanism in the Ryukyu Islands. The Dalupirup schist, exposed east of the Baguio graben in North Luzon has a date of 82.6 + 20 Ma (Wolfe 1981) establishing a Cretaceous age for some metasediments. They may be of continental origin. Whether they were derived from the margin of Eurasia (de Boer et al. 1980, Packam and Falvey 1971, Wolfe 1983) or as a part of accretions via the Philippine Sea has

PREVIOUS WORK

Gold mining in the Baguio district antedates the Spanish colonial period. Eveland (1907) was the first to make a systematic reconnaissance of the Baguio district. Since that time many papers have been written on the tectonics, stratigraphy and mineralization of the district.

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Fig. 1. Location map: Philippine region as it is today. (1) lndochina (2) East China (3) South China Sea (4) Manila Trench (5) East Luzon Trench (6) Philippine Trench (7) Philippine Sea (8) Ryukyu Trench (9) Mariana Trench (10) Japan Trench (11) Kurile Trench.

not been established. This sector was in a position to become the overriding block when subduction was initiated from the east at about 50 Ma. Volcanism was accompanied by granitic intrusions dated 49-43 Ma (Wolfe 1981). This was also a time of active volcanism in the Ryukyu arc to the north and was, prior to the establishment of the Philippine Sea plate as an entity separate from the Pacific plate. Subduction on the East Luzon Trench was initiated at about 50 Ma or slightly earlier. The subducted crust was probably of Cretaceous age. Uyeda and Ben Avraham

(1972) suggested that the western Philippine Sea is a piece of old ocean crust trapped behind the Palau-Kyushu ridge, analogous to the Bering Sea, trapped behind the Aleutian arc. Just what happened in the western Pacific in the period 50-30 Ma is not yet clear. There is a consensus that spreading occurred in what is now the Philippine Sea from 50? to 40 Ma (Louden 1976), or 46 to 42 Ma (Watts e t al. 1977). Shih (1980) extended the time of spreading throughout most of the Paleogene, 59-26 Ma. However, Louden (1976 and 1977) interpreted the 1o-

Fig. 2. Index to tectonic elements of North Luzon.

Arc magmatism and mineralization in North Luzon cation of the zone of spreading as 2-39 ° S of the equator, more or less in the position of New Guinea or Australia today, while paleomagnetic studies of North Luzon indicate that there has been little translation of this sector since the Eocene (Fuller et aL 1983). If this is correct, the Philippine Sea is not likely to have been important in the Paleogene subduction under Luzon. Spreading did overlap the time of change in direction of motion of the Pacific plate (43 Ma). Volcanism on Guam commenced at about 42 Ma (Meijer et al. 1983). It was undoubtedly related to initiation of subduction in a trench east of the island after change in direction of movement of the Pacific plate. Volcanism ceased on Guam at about 32 Ma (op. cit.) very close to the time of renewed subduction under north Luzon, thus the hiatus in dates on igneous rocks from north Luzon and cessation of volcanism in the Ryukyu Islands (Scott and Kroenke 1980), correspond closely to the first stage of volcanism on Guam. This suggests that subduction stress on north Luzon was relieved by formation of the trench east of Guam but later stress again built up to the point that subduction was renewed under north Luzon. Since Guam is thought not to have been translated (McCabe et al. 1982) and North Luzon may not have much Eocene or later translation, Louden's analysis requires transcurrent faults both west of Guam and east of Luzon of opposite sense to bring the Philippine Sea into its present position. This would also provide the vehicle to bring a series of allochthons northward to build up the Philippine archipelago (Wolfe 1983). During the Eocene, the first stage of subduction on the East Luzon Trench to develop volcanism, a reasonable rate of plate motion would be 5 cm per year. Thus 500 km of crust being subducted would penetrate to a depth of 350 km or, if oblique as little as 150 km of dip slope penetration which could yield the batholiths of the Sierra Madre. Westward subduction of the Philippine Sea plate accelerated at about 33 Ma (Wolfe 1981) and a minimum of 800 km of crust disappeared into the trench at 5 cm of movement per year over 16 Ma. Thus probably 1000km and possibly as much as 1600 km of crustal subduction took place in the two phases of the Eocene and Oligocene. The second thermotectonic regime commenced in the eastern zone of Luzon, the Sierra Madre, after a lapse of 10Ma. Quartz diorite dated 33 Ma intruded the eastern range and throughout the Oligocene plutonism migrated steadily westward with plutons dated 32.5, 31.7, 30.3, 30.0 and 29 Ma (Wolfe 1981) (Fig. 3). During this period the Cagayan basin, west of the Sierra Madre collapsed to an ultimate depth of 12 km (Tamesis 1976). By 27 Ma volcanic arcs had formed to the west of the Cagayan Valley in what is now known as the Cordillera. This is represented by two long, narrow, roughly parallel, discontinuous belts of batholiths with K-Ar dates of 27, 26, 23.2, 20.6, 19, 17.9, 17.6 and 17 Ma (Wolfe 1981). According to A. Divis (personal communication 1978), the trondhjemite of the Cordillera is strongly

81

differentiated, high in soda, low in potash with remarkably fiat REE traces at about ten times chondrites. Divis (1983) suggested that 87Sr/S6Sr ratios are a sensitive indicator of provenance. The average of four ratios from the Cordon syenite complex (Knittel 1983) is 0.70384 with an average of 23.5 Ma and 0.70374 on a Cordilleran intrusive (Divis 1983). The average of four rocks from the Agno intrusives, age 7-15 Ma (Divis 1983) gave a ratio of 0.70340. This difference of 0.0004 helps discriminate between the plutons of the East Luzon and Manila Trenches. The intrusives of the Cordillera are low in copper, zinc and other metals and no porphyry copper deposits are known from the calcalkaline rocks of the Sierra Madre or the Cordillera (Divis 1983). The Agno plutons have in contrast higher contents of these elements. The Agno plutons are confined to the Baguio graben and are all 15 Ma or younger. The youngest known Cordilleran pluton is dated 17Ma. Because of geochemical differences and younger ages, the Agno plutons should not be lumped with the older Cordilleran batholiths. The time 17-15 Ma represents the termination of subduction from the East Luzon Trench and the onset of subduction from the Manila Trench. Little is known about the area between China and North Luzon before the early Oligocene. There has been little translation of North Luzon since the Eocene (Fuller et al. 1983). The fragments of this sector which have been identified are continental in nature (Mindoro, Palawan, the Reed Bank or "dangerous area" west of Palawan and the Paracel Islands). No evidence has been presented to show that any ocean crust was present. Tapponnier et al. (1982) have developed plasticine models which suggest that Indochina was extruded southerly from Eurasia between about 35 and 17 Ma as the result of the intrusion of India into the southern border of the continent (Fig. 1). Possibly as the result of this extrusion, the South China Sea opened. The dates for the formation of new ocean crust in this zone are 32-17 Ma (Taylor and Hayes 1983). Spreading on the ridge, extrusion of Indochina and magmatism in the Cordillera of North Luzon all ceased at about 17 Ma. China began to extrude eastward at about 17 Ma (Tapponnier et al. 1982). This forced the South China Sea to move eastward, initiating subduction under north Luzon at about the same date forming the Manila Trench. This trench was terminated on the south by a trench-trench transform fault which cut across Luzon at what is now the southern end of the Cordillera. Later, after about 8 Ma, the trench extended southward (Wolfe, in prep.). By 15 Ma plutonism had commenced in the Baguio district with the first stage of the Agno batholith.

DEVELOPMENT OF THE VOLCANIC ARC

Carey and Sigurdsson (in press) illustrated the initiation and development of a marginal basin in their Fig. 1. This can be modified to illustrate development of

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Fig. 3. Section through North Luzon at latitude of Baguio. the volcanic arcs of north Luzon, as shown in Fig. 4. Only their figs 1(1) and 1(2) are pertinent to the Baguio region, since subduction and hence the spreading ceased before development of a back arc basin or marginal sea. Carey and Sigurdsson (in press) and Sigurdsson and Sparks (1978) imply that a volcanic arc forms as an extensional zone. This same concept is shown by Marsh and Carmichael (1974) in their fig. 10, which is modified

herein as Fig. 5. The volcanic arc of the Bataan lineaments related to the South Manila Trench, quite clearly illustrates this (Wolfe and Self 1983, Wolfe in prep.) A graben developed a few kilometers wide and it contains the volcanic centres (Fig. 4c). Spacing of the centers along the arc depends on the rate of subduction, hence rate of generation of magma. On the Bataan lineament this is 23 km. While not so obvious, because of erosion P

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Fig. 4(d). Fig. 4. Cartoons showing the sequence in development of the tectonic Natures of North Luzon. (a) Mid Eocene---50-43 Ma. Development of the first stage of the East Luzon Trench and the magmatism of the Sierra Madre. (b) Oligocene to Mid Miocene---33-17 Ma. During the second thermotectonic regime of the East Luzon Trench the Cagayan Valley opened as a graben and twin volcanic arcs of the Cordillera developed. (c) Mid Miocene---17-12 Ma west facing subduction in the Manila Trench with opening of the Basulo graben and intrusion of the Agno Batholith. (d) Late Mid Miocene---10-9 Ma. Intense volcanism with caldera collapse south of Bagnio followed by three pulses of mineralization. Figures (1) the East Luzon Trench which had two periods of activity (2) is a subducting ocean crust (3) Volcanism and subsequent plutonism of the Sierra Madre during the first sequence of activity. (4) Second sequence of volcanism and plutonism in the Sierra Madre. (5) Cagayan basin graben. (6) Period of volcanism and plutonism in the Cordillera, comagmatic with No. 4. (7) Development of the Bnguio graben related to the Manila Trench. (8) Manila Trench (9) Explosive eruption of the Eaguio caldera.

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in the older Baguio district, centers are as closely spaced. Elsewhere spacing of volcanic centers is noted as being 40-70km (Sigurdsson and Sparks 1978, Marsh and Carmichael 1974). In the second stage (Fig. 4c) the volcanic arc widens into two zones of volcanoes (Carey and Sigurdsson, in press). A Pleistocene example of this stage is the double volcanic arc on Leyte island, Philippines on opposite sides of the "The Philippine Fault". In North Luzon, the Cordilleran batholiths represent this stage, eroded to the subvolcanic plutons. The volcanic arc of the Manila Trench is a graben or pull-apart, eroded to the subvolcanic plutons in some sectors, but on the remnant of the Baguio plateau little erosion has occurred. Figure 4(a) shows the initiation of the volcanic arc of the Eocene East Luzon Trench. Figure 4(b) shows the results of the second pulse of magmatism from the east and the development of the graben in the Cordillera. The complication in the Oligocene is the collapse of the Cagayan Basin which reached oceanic trench depths. This type of collapse has been attributed to oblique subduction (Jacob 1980) which could have occurred during the apparent cessation of magmatism (43-33 Ma). A volcanic arc began to form about 180 km east of the Manila trench at 15-16 Ma (Fig. 4c). Typically a magma reservoir develops as the result of partial melting at a depth of about 100km. Stretching of the lithosphere

perpendicular to the direction of underthrusting occurs and extension of the crust localizes the volcanic arc in a graben. As plutons intrude into the crust, the graben tends to widen and the volcanic arc may split into two parallel segments. The twin Cordilleran batholiths of north Luzon may have formed in this manner in the Oligocene. The Rio Grande graben in New Mexico appears to be quite similar to the Baguio graben complex. It has been described (Golombek 1983) as the eastern-most graben of the Basin and Range province of the western U.S. It is 750 km from the west coast but is thought to be related to the former subduction zone west of the coast. It is also a divided volcanic arc which appears to be quite similar to the volcanic arc at Baguio but on a larger scale. The Rio Grande graben "consists of three distinct, northtrending en echelon basins that step to the right... Individual basins are about 60 km w i d e . . , the border faults are poorly exposed". He cites Muehlberger that the offsets are transform faults and Kelley that they are right echelon "relay" faults. Faults of this type do not necessarily have transcurrent motion. The western limb of the Baguio graben is located between the Black Mountain Mine in Bued Canyon and Mt Santo Tomas (Fig. 6). At this location a kilometer of vertical displacement was measured by ~ r g e SchoIcy (personal communication, 1977) down to the east. To

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Fig. 6. Showing the relationship between the relayed Baguio graben, Baguio City, the collapsed caldera and the more important mines and prospects in the district. This graben is almost exactly congruent with the "Area prospective for gold" from Fernandez and Damasco (1979) in their figure 4 and with the "crackle zone" of Gcrvasio (1979) which contains the porphyry copper bodies of the North Luzon area. Numerical key: (I) l..¢panto Gold-Copper Mine, (2) Tirad Porphyry Copper Prospect, (3) Suyoc Gold Mine, (4) Lobe and Boneng deposits of Western Minoico porphyry copper, (5) See. Nifio Porphyry Copper Deposit, (6) Angelisa Porphyry Copper Prospect, (7) Baguio Gold Mine, (8) Antamok Gold Mine, (9) Atok Big Wedge Gold Mine (10) Acupan Gold Mine, (I 1) Itogon Gold Mine, (12) Thanksgiving Zinc-Gold Mine, (13) Black Mountain Porphyry Copper Mine, (14) Philex Gold-Copper Porphyry Mine, (15) Tawi-Tawi Porphyry Copper Prospect.

86

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the south four diatremes, probably explosive breccia pipes, are located on the extension of this fault. Plutons intruded along this fault have formed the small, rich, replacement deposits of zinc, gold, copper and lead at the Thanksgiving Mine and the large, low grade disseminated copper deposit at the Black Mountain Mine. Mt Santo Tomas was active both before and after collapse of the graben as the Kennon limestone is partially enclosed within volcanics on the mountain. The eastern limit of the graben, a normal fault, is represented by the contact between the Agno plutons and the Dalupirip schist. The fault is obscured by the intrusions. The graben extended 30 km south from Baguio City to the Central Valley, where both planes of the graben are visible with a width of 15 kin. Since it offsets Pliocene sediments and contains Pleistocene volcanics, it may still be active. It may not be coincidence that the axis of the Central Valley of Southern Luzon is within the extension of the Baguio graben. This includes the small volcano and diatreme field south of Rosales and Mt Arayat, the prominent volcano east of Angeles City. These, too, are of Pleistocene age (Wolfe 1981). The graben extends 35 km north of Baguio to a point north of the Western Minolco mine. There it appears to be offset 12 km eastward by a relay fault where it is superposed upon the Cordillera, thence, extending north for another 40 km. This extension contains the Lepanto copper-gold mine, the Suyoc gold mine, the Mankayan or Tirad (Sillitoe and Gappe, 1984) copper porphyry prospect and several other significant prospects. The Lepanto enargite luzonite vein and replacement deposit is thought to be of Pleisotocene age (Gonzales 1956). This deposit is related to volcanism at Mt Data, a Pleistocene volcano located near the center of the graben. Farther north the graben becomes more obscure, but there is a suggestion that it is again relayed, to the eastern flank of the Cordillera and continues north to include the copper prospect at Mainit north of Bontoc and the Batong Buhay Mine and nearby prospects. The term Agno batholith was applied by Schafer (1954) to the early mid Miocene multiple intrusion sequence in the vicinity of the Acupan mine. Usage has extended this term to the series of north-trending batholiths in the Cordillera to the east. As shown above, the Cordilleran batholiths are somewhat older, are chemically somewhat different and appear to be the result of subduction from the east. The Agno ptutons, collectively of batholithic proportions, lie within the Baguio extensional zone, are the source of very important base and precious metals and appear to have formed as the result of subduction from the west in the Manila trench. Therefore it is recommended that the term Agno batholith be restricted to the plutons of Mid Miocene age within the Baguio extensional zone as used by Schafer (1954). Further work is necessary to detail and name the Cordilleran batholiths. The Baguio graben shown on Fig. 6 opened with the intrusion on the eastern normal fault by an elongate group of plutons which contitute the Agno batholith.

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Presumably extensive volcanism accompanied these intrusions, part of it submarine. These volcanics may constitute the "upper keratophyre" of Fernandez and Damasco (1979). These early plutons, 12-15 Ma old, were not the source of important mineralizing solutions (see Fig. 7). Adjacent to the Twin Rivers complex and extending from the vicinity of the Acupan Mine to a distance of 6-8 km south of the Philex Mine is a granodiorite pluton dated by K-Ar from outcrops near Philex as 14.8 and 15 Ma (Wolfe 1981). Shannon (1979) dated one sample from this pluton by fission track methods which came from a site adjacent to the Balatoc plug which is still thermal and Shannon noted that it had probably been thermally reset. There is also a small stock in the Bued Canyon near the Black Mountain Mine which was dated 12.4 Ma (Wolfe 1981). This is near the western limb of the graben. The nearby Lion dike, a feature on the highway south of Baguio, is dated 15 Ma (Wolfe 1981). Thus there was a period of 12-15 Ma with active volcanism and comagmatic subjacent plutonism and extension in the Baguio graben. Another period of volcanism apparently commenced about 10 Ma ago. This activity became very intense in the zone from Baguio City south for about 10 km and over a width of about 6 km. There were an estimated 50 small volcanic vents probably of the maar type scattered throughout the volcanic zones. They appear today as a series of silicified breccia pipes and breccia dikes in straight lines, such as the four pipes near the southwestern margin on the graben fault plane. Another group holds up the main high ridge containing the Country Club, Camp John Hay, Voice of America and just north of the Texas Instruments factory a large pipe which was quarried for silica. Another group of breccia pipes is found in the eastern

Arc magmatism and mineralization in North Luzon part of Baguio City in the Mines View Park areas. This line continues down to the Tuding road which has been diverted around a breccia pipe. This pipe has an extensive alteration halo, as discussed below. The line of pipes continues on down past the hotel in the valley. While more pipes have been identified in the western and northwestern sector, there are several scattered throughout the central part of the district and extending to the eastern margin of the graben. The Balatoc pipe at the Acupan Mine has a strongly silicified eastern border but the main pipe appears to be much younger and is still thermal. This may be a nested pipe, with the initial eruption at the time of the pyroclastic event and the second stage occurring in the Pleistocene. The pipes which are now readily visible are strongly silicified and apparently had a substantial pyrite content. T h e pyrite in the zone of weathering has since been oxidized to limonite and hematite. Many of these pipes probably erupted as maar-type volcanoes and collectively they represent a cataclysmic caldera eruption. Since within a graben, it is a volcanotectonic depression. Since only a small remnant of the former high ground around Baguio remains, most of the pyroclastics have been eroded. Along the highway north of Baguio from Trinidad to Acops Place, a pyroclastic remnant of the caldera extrusives has been mapped as the Picopyroclastic member of the Klondyke formation. Typically the eruption of a caldera exhausts the volatile mineralizer components of the magma chamber. Of the magma itself, probably not more than 10% is erupted (Smith and Shaw 1973). It appears that volatiles must have rapidly reaccumulated and permeated the ruptured roof of the batholith. The many diatremes provided ready passage of the solutions and a silicapyrite flooding occurred in many of the breccia pipes. While the pipes themselves were the plumbing, the explosion of the breccias intensely fractured the surrounding ground and solutions seeped out into it, creating large halos of pyritic and argillic alteration. The best exposure of this type of alteration is along the Tuding road at the City Limit. The breccia pipe outcrops between the Tuding road and the Mines View Park road. Below the Tuding road, erosion has exposed 300-400 m of alteration, weathered to the yellow and seal brown colors typical of the phyllic zone around porphyry copper deposits. Assuming that the date of 9.6 Ma on the volcanics on the Naguilan Road represents the time of the caldera eruption, the intense hydrothermal alteration of the district must have taken place between 9 and 10 Ma. This alteration zone is so intense that it is readily visible on satellite photographs (Divis 1983) as an elliptical zone 13 x 7.5 km in size (Fig. 6).

STRATIGRAPHY OF THE BAGUIO REGION

A brief discussion of the stratigraphy of the district is pertinent to the tectonics and mineralization. Balce et al. (1980, p. 270) said, "The proliferation of varying strati-

87

graphic schemes is indeed, the most awesome problem one has to face in understanding the geology of the Baguio district". At least nine different stratigraphic classifications have been published. Figure 7 as compiled from several of these sources. Possibly part of the differences have arisen from failure to recognize the graben and caldera which have markedly different sedimentation within them and marginal to them. Pre- Tertiary

The Oligocene Cordilleran batholiths intruded a metamorphic complex composed of schist, quartzite, slate, some marble and pillow lavas. The only known date on these rocks is 82.6 + 20 Ma B.P. (Wolfe 1981), probably marking a regional metamorphic event. The name Dalupirip Schist has been given to this group of formations (Balce et al. 1980). Divis (1983, p. 181) stated, "The rocks are generally of greenschist or upper epidote amphibolite facies . . . . Superimposed contact metamorphism and hydrothermal activity associated with Miocene intrusions frequently overprints the regional metamorphic mineralogy". The Oligocene intrusives undoubtedly had a similar effect. These rocks may represent continental margin or a marginal arc prior to opening of the South China Sea. Pugo

This formation name is retained here because it is well known, although the lower boundary is not agreed upon. Balce et al. (1983) mentioned Eocene and Oligocene fossils from this formation. It is reported to be Cretaceous-Paleogene in age. Fernandez and Damasco (1979) have made the most extensive study of the rocks generally included in this formation, both underground in the Antamok mine and on the surface. However, they did not use the formation name. Instead they defined the lower, mid and upper keratophyre series, of Eocene and Oligocene age. Balce et al. (1980) included all preMiocene units in this formation as did Schafer (1954) and Pefia (1970). Others generally assigned these rocks to "Basement Complex". In attempting to correlate the keratophyres to dated events in the region (this study), the lower keratophyre is assigned to the Cretaceous-Paleogene (100-50Ma), the middle keratophyre to the Oligocene Cordilleran plutonic stage (29-17 Ma) and the upper keratophyre brought up to the Mid Miocene (15-12 Ma) to correlate with the Agnointrusives. Fernandez (personal communication, 1985) does not object to this dating. The Lower Keratophyre is conglomeratic at the base with copper in some places to 0.5%. This is overlain by hybridized volcanics with some pillow lavas and andesite flows. The middle member included the Wildcat sediments of the Antamok mine, locally strongly replaced by copper. This sector is strongly sericitized and silicified. The upper member is a variety of wackes and shales including some red beds. The Mid Keratophyre is composed of massive lava flows, predominantly andesite, with some dacite pyroclastics. Sedimentary intercalations are composed of

88

tonN A WoR~

thick wackes, hematitic mudstones, ferruginous volcanic conglomerate with lenses of marbilized reefal limestone. The limestone of this unit is the host to the rich gold zinc replacement deposit at the Thanksgiving Mine in Bued Canyon (Fernandez and Damasco 1979). Gold content of this unit is anomalously high in the country rock and veins through this unit are more productive than in other formations (op. tit.) The Upper Keratophyre is probably transitional with the middle unit. It consists " o f a thick series ot" alternating agglomeratebreccia and massive flows . . . . The sediments are predominately volcanic conglomerates, of distinctly redpurple color and minor wacke-shale-silt stone interbeds . . . . "" (op. cir. p. 1857). No thickness measurements have been given tbr the Pugo-keratophyrc formation. The individual units vary drastically in thickness and lithology in short distances. In general the formation is 500 m or more thick, Zigzag or ""Upper Zigzag ~"

The name of the formation comes from the section of the Kennon Road south of Baguio where the road "zigzags" out of the canyon. At this location the lower third of the formation includes several andesitic tufts and welded tufts and at one third of the thickness from the bottom the first hornblende diorite pebbles of the Agno plutons appear. The tufts undoubtedly represent the volcanics comagmatic with the intrusives, the beginning of the volcanic activity related to the Manila Trench. The Baguio graben had started to form and channeled much of the sediment load southward. The upper two thirds of the formation is conglomeratic. The thickness is 1700 m (Pefia and Reyes 1970). To the north of Baguio it is 500 m thick and of Early Miocene Age, based on, "Lower to Middle Miocene and reworked Eocene foraminifera" (Balce et al. 1980. p. 276). The h'ennon Formation

The Kennon Formation conformably overlies the Zigzag, The lower member is the Kennon limestone which outcrops prominently in the Bued Canyon south of Baguio. It consists principally of calcarenites and calcirudites of Mid Miocene age. It represents a time of quiescence of volcanism and minimal erosion. Balce et al. (1980) consider the limestone on Mt Mirador to be part of this formation. Most authors, for example Pefia (1970) consider them to be two different formations. Here they are classed as two separate units. Thickness ranges up to 240 m. Locally it is conformably overlain by the Twin Peaks member composed of sandstone and shale of Mid Miocene age. Balce et al. (p. 273, 1980) reviewed the extensive study of fossils from the formation and concluded the formation is " o f early Mid Miocene age". K h m d y k e [brmation

Unconformably over all formations in the Baguio area is the Pico pyroclastic member of the KIondyke, ranging

from 250 to at least 500 m thick. It represents massive renewal of volcanism in the area, estimated to have occurred about 9.6 Ma, early Late Miocene. The i0rmation of the Baguio caldera represents a part of Ihis activity, Extensive erosion followed with the thick co~v glomerate member forming north, south and ~est of Baguio. Balce et al. (1980) measured 1798 m in the Bued Canyon but they note it may be appreciably thicker than this. A portion of this may represent initial dip (M. Dean Williams, personal communication, 1966!, hence measurement of thickness of all sedimentatry units may exceed the true thickness of the lbrmation. ,4mlang ,/ormation

Flanking the uplifted Cordillera and the Baguio volcanic arc is the marine Late Miocene Amlang formation (Lorentz 1984). It is at least 1600 m thick, composed of fine clastics with a resistant sandstone composed of turbidites. It has a gradational contact with the underlying Klondyke, but is unconformable with the overlying Cataquintingan. Cataquintingan f o r m a t i o n

The Pliocene Cataquintingan ranges from 900 m in the south to 2600 m thick in the north, composed of mixed marine clastics, tending to more conglomeratic in the lower portion and more tuffaceous in the upper zone (Lorentz 1984). The Amlang and Cataquintingan formations were included in the Rosario formation by Balce et al. (1980) and earlier authors. Mirador Limestone

At various times local coralline reefs developed flinging the growing archipelago. Most of these were of restricted occurrence. One such reef forms very prominent hills in the western part of Baguio City, at Mt Mirador, from which the formation gets its name. The Late Miocene to Pliocene age seems to be most widely accepted. Corby (1951) classed it as Early Miocene and older than the Kennon. Fossils are scarce and the age is quite controversial and cannot be resolve here. Pefia (1970) classed it as Pliocene, Fernandez and Pulanco (1967) called it Late Miocene to Pliocene. Balce et al. (1980) interpet it as being a reefal facies of the Kennon thus Mid Miocene. Quaternary alluvium

The Quaternary was a time of dramatic, rapid uplift, with development of nearly two kilometers of relief over very short distances. The central part of Baguio, now at about 1400 m is a remnant of a mature surface perched above deep canyons, the surface rapidly shrinking. Denudation and the works of man have accelerated this erosion. Uplift was spasmodic with development of a series of terraces which contain gravel. The major gold mineralization of the district occurred during the Pleistocene and Fernandez and Damasco (1979) have cor-

Arc magmatism and mineralization in North Luzon

89

related ore shoot development to the prominent Pleis- correlated with terraces representing stages of erosion tocene terraces. Because of very steep stream gradients, during the Pleistocene. valleys are degrading and very little stream alluvium is At some time in the Pleistocene plutons again invaded found. Differential movements resulted in development the Baguio caldera. The dacite porphyry at Itogon was of small basins which became lakes until overflow outlets dated 1.7 Ma by fission track methods (Shannon 1979). eroded and drained the valleys. The town of Trinidad, A short distance from this pluton, at Acupan, a phreatonorth of Baguio, is situated on lacustrine sediments of magrnatic eruption took place forming the Balatoc breccia pipe. The breccia fragments include many types Pleistocene age. of volcanics, quartz-diorite mineralized with chalcopyrite to ore grade, occasional serpentine xenoliths INTRUSIVES AND RELATED MINERALIZATION and a few carbonized logs of trees which collapsed into the maar crater. Accompanying the collapse of the maar Important gold mines of the Baguio district are crater, tectonic breccias were formed around the periphAntamok, Acupan, Atok-Big Wedge, Itogon, all in ery of the pipe. Some of these tectonic breccias are richly production, and Baguio Gold, being re-evaluated. A mineralized with gold, as are the faults which intersect large, low-grade gold deposit at the Acupan mine is in the pipe The pipe has been chloritized and carbonated production. Major copper mines are Philex, Black but only low gold values are found within the pipe itself Mountain, Santo Nifio, Western Minolco, all dissem- (Fernandez et al. 1979). The Pleistocene was the time inated type, and Lepanto, a large vein and replacement of most intense gold mineralization throughout the country. deposit. Gervasio (1979) identified what he called a "crackle zone" west of the Cordilleran batholiths extending for PHYSIOGRAPHY some distance north and south of Baguio in which 22 bodies (now 25 including relayed extensions) of dissemUplift in northern Luzon appears to have been spasinated copper mineralization have been found. Fernmodic. By latest Miocene or Early Pliocene a mature andez and Damasco (Fig. 4, 1980) showed the "area land surface had formed at a lower elevation. There was prospective for gold exploration" which approximately rapid uplift in the Pleistocene and Balce et al. (1980) coincides with Gervasio's (1979) "crackle zone". Both of delineated the eroded remnant of a land surface. Study these trends fall within what is here called the volcanic of the breccia pipe on the highway south of Baguio in arc of the North Manila Trench, an extensional zone, a a quarry, located just north of the Texas Instruments garben or pull-apart, active from 15 Ma to the present, plant, showed that the tephra layers formed during the shown in Fig. 6. phreatomagrnatic explosion of the pipe are still intact. The area immediately south and east of Baguio is Accretionary lapilli which were formed from the ash intensely altered and contains many explosive breccia cloud, were also found. The wave of silicification which pipes and extensive volcanics. These are believed to be invaded the pipe extended out into the tephra layers and evidence of a caldera eruption. Since it is within a even replaced the accretionary lapilli. The presence of graben, it should be classed as a volcano tectonic the tephra layers indicates that there has been little depression. erosion of this portion of the ancient upland since the The Agno Batholith intruded the Baguio graben in the caldera eruption. Mid Miocene and probably extends for a distance of about 40 km north to south with a width of 7-8 km. Only a few of the cupolas have been exposed by erosion DISCUSSION near Baguio but it is more extensively exposed east of the Philex Mine. A central, volatile-rich "bubble" developed A "pure" island arc is always generated on ocean and broke, resulting in the caldera formation south of crust. Thus the basement is an ophiolite which is only Baguio. Porphyry copper mineralization bracketted the exposed when faulting raises the basic rocks and erosion time of caldera eruption. The silica-pyrite flooding event removes the sedimentary cover. Many times only the occurred shortly after the caldera eruption. pillow basalts are exposed on the surface. Hawkins and There were three distinct pulses of mineralization in Evans (1983) have interpreted the Zambales ophiolite the district, the first in the Late Miocene, described which makes up the Zambales mountains as the roots of above and the second in the Late Pliocene. Dates on an island arc. Hayes and Lewis (1984) project the the volcanism of Bukod, Benguet a short distance east Zambales ultramafic northward along the coast of Luof Baguio show 6.1 Ma for the volcanics, 5.6 Ma on the zon, coming on shore near the northwestern tip of the quartz diorite and three dates of 3.6 Ma (Wolfe 1981) on island, a distance of 450 km. Why this long, narrow the alteration of this quartz-diorite. The alteration repre- ophiolite formed along the full length of Luzon is not sents the second pulse, the time of formation of the known. The simplest explanation is that the development Tawi-Tawi porphyry copper deposit (Wolfe 1981). The of the Manila Trench resulted in uplift and eastward tilt third pulse, in the Pleistocene, was the most important of a block of crustal rocks. Karig (1983) emphatically time of gold mineralization. Fernandez et al. (1979) have opposed this origin. Instead he suggested the ophiolite is shown that the deposition of gold at Baguio can be an allochthon that moved into the area as a wedge S.E.A.E.S. 2/2--C

c)(~

JOHN A. WOI.FI

between the South China Sea and Luzon from the Celebes Sea, a distance of about 1000 k m In his analysis injection of the ophiolite occurred when Luzon was about 500 km east of its present location and was shoved westward as Luzon moved over the Manila Trench. subducting crust of the South China Sea. The idea that the Philippines is the active block in the subduction of the South China Sea crust is widely accepted, for example (Hamburger et al. p. 4, 1983) "~. • collision of the northern end of the Philippine magmatic arc with the passive continental margin of Asia". The colliding zone is inferred to be the North Luzon Ridge and Taiwan, and this is considered to be the reason why subduction is halting in the Manila Trench. Were the Philippines the active block, it is probable a left lateral transcurrent fault would severe the North Luzon Ridge and the rest of Luzon would have continued westward. The passiveness of Asia has recently been questioned. Modelling studies of Tapponnier et al. (1982) suggest that the intrusion of India into southern Asia has resulted in the extrusion of east China on the right lateral Red River fault and the left lateral Altyn Tagh fault complex. Coupled with the South China Sea crust, this block has moved eastward commencing 17 Ma, initiating subduction under passive Luzon forming the Manila Trench. Figure 8 shows the relationships around the Philippines in Mid Miocene. By Late Quaternary time, the stress within Asia has progressed farther north and extrusion of east China has nearly halted, resulting in only minor and residual tectonic activity under North

Fig. 8. Configuration of the South China Sea and Philippines at about 15 Ma (I) South China Sea (2) Manila Trench (3) Luzon Transform Fault (4) Sangihi Trench (5) Palawan Allochthon (6) Indo China (7) Extruding East China.

Luzon which relates to the Manila Trench. Xie and Peng (1982) have independently proposed the extrusion of eastern China in the Neogene. While a cratonic element can overide a trench, subduction of an active ocean plate is more common. There is no evidence that any ocean crust existed between Asia and North Luzon prior to the opening of the South China Sea. This factor has been generally overlooked. All remnants of the older rocks are continental in nature (North Palawan, Reed Bank or "dangerous area" and Mindoro). The area west of the South China Sea is altered continental sediments (Holloway 1982). If the South China Sea is closed back to its original position, these allochthonous blocks move back to the China coast. Nuclear Luzon, the Cretaceous and older metasediments which underlie the Cordilleran and Sierra Madre area can also be translated from an adjacent position, but only moving half as far as the other allochthons. This would conform in part to de Boer et al. (1980), Holloway (1982) and Wolfe (1983). Subduction could not have been initiated until after crust of the South China Sea had formed and the sea had widened to a few hundred kilometers. Spreading started about 32 Ma (Lewis and Hayes 1980) and if 80 km of crust were formed per Ma (half spreading rate of 4 cm per year), the earliest time that subduction would be likely would be about 22 Ma when 800 km of ocean crust had formed. Since intrusives were forming in the Cordillera by 30Ma (Wolfe 1981), 180km to the east, it would be impossible for the Cordilleran batholiths to be related to the Manila Trench. The Baguio district, the volcanic arc related to the Miocene to Recent east-dipping Benioff of the northern sector of the Manila Trench is a text book example of the entire consuming margin sequence of the plate tectonic theory. It is a compound problem, complicated by the two-stage Paleogene east Luzon trench and its west-dipping Benioff zone, and the younger, eastdipping North Manila Trench, both of which extended beneath the Baguio zone, While there is general agreement on most of the major features, there is still disagreement on several details. Points of controversy are: (1) Do the Cordilleran batholiths relate to the East Luzon Trench or the Manila Trench? The thesis of this paper is that they relate to the East Luzon Trench, based on the steady decrease in age from east to west from 33 to 17 Ma in the Cordillera. Were the Manila Trench the source of these plutons, the oldest rocks would have to have been formed in the west and the youngest in the east. The small aceretionary prism of the Manila Trench represents only about 10 Ma (Hayes and Lewis 1984) not 35 Ma. Spreading ceased in the South China Sea, at about 17 Ma (Lewis and Hayes 1983). This study follows the idea of Tapponier et al. (1982) that the extrusion of East China beginning at the close of spreading in the South China Sea, 17 Ma, was the force that initiated subduction of the South China Sea under North Luzon. Thus the volcanic arc could not have been initiated until about 15 Ma. As discussed in the text, rock geochemistry

Arc magrnatism and mineralization in North Luzon of the intrusives (differences in Sr ratios and in metal content) suggest different subduction sources for the two batholithic systems. According to Balce et al, (1980) the Manila Trench formed in the late Oligocene and they relate the Cordilleran batholiths as well as the Agno plutons to the early location of the Manila Trench at the North Luzon Trough. Seismic lines across the Manila Trench accretionary prism and forcarc basin (Hayes and Lewis 1984 and Lewis and Hayes 1984) do not support the idea that the North Luzon trough was formerly the locus of subduction. This will eventually be resolved when the Cordilleran batholiths are studied in detail after many more radiometric dates and complete chemical analyses are run on the plutons on both sides of the assumed boundary. (2) Volcanologists in general seem to accept the idea that a volcanic arc is formed as an extensional zone, that it is localized by extension (graben formation) and the extension can broaden into a back arc basin and eventually into a marginal sea. It is proposed here that there are several examples in the Philippines where extension has progressed to the double volcanic arc stage, the Baguio District being one of them. The southern Luzon volcanic arc of the Manila Trench, the Bataan Lineament (Wolfe and Self 1983), has only widened to the single volcanic arc stage and is several million years younger than the North Manila Trench (Wolfe, in prep.) (3) Was there a volcanotectonic depression (caldera) at Bagulo about 9.6 Ma? The large number of intensely

silicified explosive breccia pipes aligned as if following extensional fractures is strong evidence that explosive volcanism of the type most common in cratonic areas occurred, and logically would have resulted in collapse of the caldera. The extensive zone of hydrothermal alteration, visible on satellite images (Divis 1983) coincides with the postulated caldera. The Pico volcanics north of Baguio can be most easily explained as a remnant of the caldera eruption. They thicken to 500 m in the vicinity of Baguio (Balce et al. 1980). Because of the rapid uplift to 1500-1800m above sea level and extensive erosion, much of the evidence of the caldera has been destroyed. (4) Can porphyry copper mineralization occur in an extensional zone? In this case the postulated graben coincides with what Gervasio (1979) called a "crackle zone" which localized 22 bodies of porphyry copper mineralization and with the zone designated by Fernandez and Damasco (1979) as most prospective for gold mineralization. Sillitoe (1980) presented an argument that in cratonic regions porphyry copper occurrences are unlikely in extensional zones. If the argument in point 2 above is accepted for an island arc system, mineralization within such a zone should be accepted. Probably a major part of the difficulty in unravelling the stratigraphy of the Baguio district results from the great difference in the nature of sediment within the graben and outside of it. Pre-graben (15 Ma) the source of sediment was from the Cordillera which may have

70 60

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Cretaceous and possibly older rocks of a continental nature are present in the Sierra Madre Philippines per ~'e did not exist. 122:E longitude broke into a west-dipping subduction zone east of Luzon. Volcanic arc developed in Sierra Madre. Granitic intrusions. Philippine Sea plate largely moving northward but enough oblique westerly motion to generate magma. No record of activity. Mariana Trench had formed east of Guam, absorbing stress from changed direction of Pacific plate. Philippine Sea plate probably moved di.. rectly north, transcurrent motion against primitive [.u zon. Renewed east-lacing subduction of Philippine Sea plate under Luzon north of 15 30', a second stage. At least 1000 km and possibly 1600 km of Mesozoic ocean floor were subducted, predominantly westward motion. Formation of volcanic arc in position of Cordillera, collapse of Cagayan Valley. (Subduction also in the Sanghi Trench in southern Philippines in position of Agusan Valley.) Impact of India on Asia resulted in extrusion of Indochina. This stress resulted in pull-apart, forming South China Sea. Major part of Zambales ophiolite tbrmed. Spreading in South China Sea had halted by 17 Ma. As India intruded farther into Asia, extrusion of lndochina halted and eastward extrusion of east China commenced. This resulted in initiation of subduction eastward under Luzon and formation of the Manila Trench. However, subduction was limited on the south (about 16 30'N) by the Luzon transform, a trench trench transform joining the early stage Philippine trench to the Stage 1 North Manila Trench. A volcanic arc and the Agno batholiths developed west of the Cordillera of North Luzon and in this stage 400 km of South China Sea floor were subducted A volcanotectonic depression (graben) formed east and south of Baguio on the apex of the Agno batholith. Disseminated copper mineralization took place in the Baguio graben Zambales began to emerge. A segment of the Philippine Fault zone mowng letl lateral cut the Luzon transform, breaking the connection between the two trenches, permitting the Manila Trench to extend south. In stage 2 (south) about 300km o[ South China Sea were subducted. The w~lcanic arc and graben of the south limb of the Manila Trench began to form at about 7 Ma with porphyry copper mineralization at San Marcelino. A second pulse of disseminated copper mineralization occurred in the late Ptiocene east of the Baguio graben, within the graben itself and on the relayed northern extension of the graben near Lepanto. A third stage constituting the major period of gold mineralization occurred within the graben in the Pleistocene. The Manila Trench appears to be dying and reversing to the east side of Luzon

been at times a rather high land mass, shedding volcanics and elastics into the sea around Baguio. Until Late Middle Miocene the volcanic arc consisted only of volcanic islands. Before the caldera eruption (about 10Ma) the area was uplifted and eroded. After the eruption and collapse of the caldera, sedimentation outside the caldera was conglomeratic near shore, with finer elastics at more distant points. Within the caldera however, it was mixture of agglomerates, lahars, occasional ash flows intermixed with volcanoclastics. This zone may represent the upper keratophyre of Fernandez and Damasco (1980). CONCLUSIONS I. N o author has presented evidence that any ocean

crust existed between North Luzon and China prior to opening the South China Sea, commencing 32 Ma. 2, Subduction could not have started in the Manila Trench before 22 Ma. tt did noi commence until about 17 M a

13. Batholiths in the Luzon Cordillera are related to the second thermotectonic regime of the East Luzon Trench, 33- 17 Ma, not the Manila Trench. 4. The volcanic arc of the north segment of the Manila Trench formed in a graben extending north and south of Baguio, commencing to collapse about 15 Ma. 5. The Agno batholith intruded into the broad Baguio graben from 15 to 12 Ma with plutons continuing to intrude as late as the Pleistocene. 6. Renewed volcanism and intrusion I0-8 Ma represented the first stage of porphyry copper mineralization. 7. A volcano tectonic depression formed about 9.6 Ma resulting in caldera collapse, south and east of Baguio, within the graben. 8. Rapid reaccumulation of volatiles resulted in intense hydrothermal alteration of the caldera zone, now visible on satellite photos. This represents the continuation of (6). 9. There are 22 known bodies of porphyry copper mineralization and the largest gold district in the Orient within the Baguio graben complex. Three additional porphyry copper deposits are found in the relayed northern extension of the graben 10. A second pulse of mineralization occurred about 4-3 Ma and extended beyond the graben. i J. The gold mineralization was largely confined to the Pleistocene and to the graben. 12. The graben was relayed eastward and superposed on the Cordillera at the north end, probably in the Pliocene, resulting in formation of Pleistocene volcanism and gold and copper deposits.

REFERENCES Balce, G. R., Encina, R. Y., Momongan, A. and Lara, E. 1980. Geology of the Baguio District and implications on the tectonic development of the Luzon Central Cordillera. Geol. Paleontol. SE Asia 27, 265--287. Carey, S. and Sigurdsson, H. in press. A model of volcanogenic sedimentation in marginal basins. Q. J. Geol. Soc. Lond. presented in 1982. Corby, G. W. et al. 1951. Geology and oil possibilities of the Philippines. Department of Agriculture and Natural Resources Tech., Bul. No. 21. de Boer, J., Odom, L. A., Ragland, P. C., Snider, F. G. and Tilford, N. R. 1980. The Bataan orogene; eastward subduction, tectonic rotations, and volcanism in the western Pacific (Philippines). Tectonophysics 67, 3376-3389. Divis, A. F. 1983. The geology and geochemistry of Philippine porphyry copper deposits. In: The Tectonic and Geologic Evolution of Southeast Asian Seas and Islands, Part 2. American Geophysical Union Monograph 27 (Edited by Hayes, D. E.), pp. 173-216. Fernandez, H. E. and Damasco, F. V. 1979. Gold deposition in the Baguio gold district and its relationship to regional geology. Econ. Geol. 74, 1852-1868. Fernandez, H. E., Damasco, F. V. and Sangalang 1979. Gold ore shoot development in the Antamok mines, Philippines. Econ. Geol. 74, 606--627.

Arc magrnatism and mineralization in North Luzon Fernandez, J. and Pulanco, D. 1967. Reconnaissance geology of northwestern Luzon, Philippines. In: 2nd Geol. Convention and 1st Symposium on the Geology of the Mineral Resources of the Philippines and Neighboring Countries, Proceedings, vol. 1, p. 35. Geol. Soc. Philippines, Manila. Fuller, M., MeCab¢, R., Williams, I. S., Almasco, J., Encina, R. Y., Zanoria, A. S. and Wolfe, J. A. 1983. Paleomagnetism of Luzon. In: The Tectonic and Geologic Evolution of Southeast Asian Seas and Islands, Part 2, American Geophysical Union Monograph 27 (Edited by Hayes, D. E.) pp. 79-94. Gervasio, F. G. 1979. Tectonic setting of porphyry copper deposits of the Philippines. J. Geol. Soc. Philipp. 23, (3), 1-12. Golombck, M. P. 1983. Geology, structure and tectonics of the Pajarito fault zone in the Espafiola basin of the Rio Grande rift, New Mexico. Bull. Geol. Soc. Am. 94, 192-205. Gonzales, A. 1956. The Lepanto Copper Mine. In: Copper Deposits of the Philippines (Edited by A. R. Kinkel et al.) Philippine Bureau of Mines SPS No. 16. Hamburger, M. W., Cardwell, R. K. and Isacks, B. 1983. Seismotectonics of the Northern Philippine island arc. In: The Tectonic and Geologic Evolution of Southeast Asian Seas and Islands, Part 2 American Geophysical Union Monograph 27 (Edited by Hayes, D. E.) pp. 1-22. Hayes, D. E. and Lewis, D. 1984. A geophysical study of the Manila Trench, Luzon, Philippines, 1, Crustal structure, gravity and regional tectonic evolution. J. Geophys. Res. 89, 9171-9195. Hawkins, J. and Evans, C. 1983. Geology of the Zambales range, Luzon, Philippine Islands. Ophiolite derived from an island arc-back arc basin pair. In: The Tectonic and Geologic Evolution of Southeast Asian Seas and Islands, Part 2, American Geophysical Union Monograph 27 (Edited by Hayes D. E.) pp. 95-123. Holloway, N. H. 1982. North Palawan block, Philippines--its relation to Asian mainland and role in evolution of the South China Sea. Bull. Am. Ass Petrol. Geol. 66, 1355-1383. Jacob, K. H. 1980. Oblique subduction and rifting along ocean-continent transform boundaries. EOS Abstr. 61, 358. Karig, D. E. 1983. Accreted terranes in the northern part of the Philippine archipelago. Tectonics 2, 211-236. Knittel, U. 1983. Age of the Cordon syenite complex and its implication on the mid-Tertiary history of North Luzon. Philipp. Geol. 37 (2), 22-31. Lewis, S. D. and Hayes, D. E. 1980. The structure and evolution of the central basin fault, West Philippine Basin. In: The Tectonic and Geologic Evolution of Southeast Asian Seas and Islands, American Geophysical Union Monograph 23 (Edited by Hayes, D. E.) pp. 89-104. Lewes, S. D. 1983. The tectonics of northward propagating subduction along eastern Luzon, Philippine Island. In: The Tectonic and Geologic Evolution of Southeast Asian Seas and lslands: Part 2, American Geophysical Union Monograph 27 (Edited by Dennis E. Hayes) pp. 57-78. Lorentz, R. A. Jr 1984. Stratigraphy and sedimentation of Late Neogene sediments on the southwest flank of Luzon Central Cordillera, Philippines. J. Geol. Soc. Philipp. 38 (2), 1-24. Louden, K. E. 1976. Magnetic anomalies in The West Philippine Basin. In: American Geophysical Union Geophysical Monograph 19, pp. 253-267. Louden, K. E. 1977. Paleomagnetism of DSDP sediments, phase shifting of magnetic anomalies and rotations of the West Philippine Basin. J. Geophys. Res. 82, 2989-3002. Marsh, B~ D. and Carmichael, I. S. E. 1974. Benioffzone magmatism. J. Geophys. Res. 79, 1196-1206. McCabe, R., Fuller, M., Williams, I. S., Almasco, J., Encina, R. Y., Zanoria, A. S. and Wolfe, J. A. 1982. Paleomagnetism of Luzon. EOS Trans. Am. Geophys. Un 63, No. 45, 913. Meijer, A., Reagan, M., Ellis, H., Shafigullah, M., Sutter, J., Damon, P. and King, S. 1983. Chronology of volcanic events in the eastern

93

Philippine Sea. In: The Tectonic and Geologic Evolution of Southeast Asian Seas and Islands: Part 2: American Geophysical Union Monograph 27, pp. 349-359. Morgan, W. J. 1972. Plate motions and deep mantle convection. In: Studies in Earth and Space Sciences (Hess volume), (Edited by G. R. Shagam et al.). Geol. Soc. Am. Mem. 132, 7-32. Packham, G. H. and Falvey, D. A. 1971. An hypothesis for the formation of marginal seas in the Western Pacific. Tectonophysics 11, 79-109. Pefia, R. 1970. Brief geology of a portion of the Bagnio mineral district. J. Geol. Soc. Philippines 24, No. 4, 41-43. Pefia, R. E. and Reyes, M. V. 1970. Sedimentological study of a section of the "Upper Zigzag" formation along Bued River, Tuba, Bengnet. J. Geol. Soc. Philipp. 24, No. 1, 1-19. Schafer, P. A. 1954. Some phases of the geology of the Baguio District. Mining Newsl. Manila, 5, 5-7, 33-35. Scott, R. and Kroenke, L. 1980. Evolution of back arc spreading and arc volcanism in the Philippine sea: interpretation of Leg 59, DSDP reports. In: The Tectonic and Geologic Evolution of Southeast Asian Seas and Islands. American Geophysical Union Monograph 23, 281-291. Shannon, J. R. 1979. Igneous petrology, geochemistry and fission track ages of a portion of the Baguio mineral district, northern Luzon, Philippines, Thesis T-2185, Colorado School of Mines, Golden, Colorado, pp. 1-173. Shih, T. 1980. Marine Magnetic anomalies from the western Philippine Sea: implications for the evolution of marginal basins. In: The Tectonic and Geologic Evolution of Southeast Asian Seas and Islands, American Geophysical Union Monograph 23 (Edited by D. E. Hayes), pp. 49-76. Signrdsson, H. and Sparks, S. R. J. 1978. Lateral magma flow within rifted Icelandic crust. Nature 274, 126-130. Sillitoe, R. 1980. Are porphyry copper and Kuroko-type massive sulfide deposits incompatible? Geology 8, 11-14. Sillitoe, R. and Gappe, I. Jr 1984. Philippine porphyry copper deposits: geologic setting and characteristics. CCOP Project Office RAS/81/120, pp. 1-88. Smith, R. L. and Shaw, H. R. 1973. Volcanic rocks as geologic guides to geothermal exploration and evolution. EOS Trans. Am. Geophys. Un. 34, 1213. Tamesis, E. V. 1976. The Cagayan Valley basin: a second exploration cycle is warranted. In: Offshore Southeast Asia Conference. Tapponnier, P., Peltzer, G., Le Dain, A. Y. and Armijo, R. 1982. Propagating extrusion tectonics in Asia: new insights from simple experiments with plasticine. Geology 10, 611-616. Taylor, B. and Hayes, D. E. 1983. Origin and history of the South China Sea. In: The Tectonic and Geologic Evolution of Southeast Asian Seas and Islands: Part 2, American Geophysical Union Geophysical Monograph 27 (Edited by Dennis E. Hayes) pp. 23-56. Uyeda, S. and Ben Avraham, Z. 1972. Origin and development of the Philippine Sea. Nature Phys. Sci. 240, 176-178. Watts, A. B. 1977. Sea floor spreading in marginal basins of the Western Pacific. Tectonophys. 37, 167-181. Wolfe, J. A. 1981. Philippine geochronology. J. Geol. Soc. Philipp. 35, 1-30. Wolfe, J. A. 1983. Origin of the Philippines by accumulation of allochthons. J. Geol. Soc. Philipp. 37 (3), 16-33. Wolfe, J. A. and Self, S. 1983. Structural lineaments and Neogene volcanism in Southwestern Luzon. In: The Tectonic and Geologic Evolution of Southeast Asian Seas and Islands: Part 2 American Geophysical Union Monograph 27 (Edited by D. E. Hayes) p. 157-172. Xie, Q. and Peng, F. 1982. On the evolution of the Ryukyu arc and the basin of the East China Sea. In: Proceedings, Fourth Regional Conference in the Geology of Southeast Asia, pp. 41-49. Geological Society of the Philippines.