The Paracale Intrusion: geologic setting and petrogenesis of a trondhjemite intrusion in the Philippine Island Arc

Journal of Southeast Asian Earth Sciences, Vol. 1, No. 4, pp. 235--245, 1986 Printed in Great Britain

0743-9547/87 $3.00 + 0.00 Pergamon Journals Ltd.

The Paracale Intrusion: geologic setting and petrogenesis of a trondhjemite intrusion in the Philippine Island Arc UWE GIESE Institut fiir Geologic und Pal/iontologie der RWTH Aachen, Wiillnerstr. 2, D-5100 Aachen, F.R. Germany ULRICH KNITTEL* Institut fiir Mineralogie und Lagerst/ittenlehre der RWTH Aachen, Wiillnerstr. 2, D-5100 Aachen, F.R. Germany

and ULRICH KRAMM Institut fiir Mineralogie, Corrensstr. 24, D-5100 Mfinster, F.R. Germany (Received 1 July 1985; accepted for publication 21 October 1986) Abstract---On the northern shore of Camarines Norte (Southern Luzon, Philippines) a tonalite/trondhjernite intrusion hosted by an ultramafic complex is exposed. The ultramafic complex is composed of metamorphic, depleted harzburgite and is considered as part of an ophiolite-sequence. The trondhjemites, which comprise the Paraeale Intrusion, may have formed by partial melting of tholeiitic materials under high water pressure. The magma may have been intruded as semi-solid plagioclase cumulate which resulted in a pronounced flow lamination of the rocks.

INTRODUCTION

a strong tectonic overprint, which renders a single K-Ar age unreliable. Recently it has been postulated that the eastern part of the Philippine Mobile Belt is an accreted island arc (Talaud-Mindanao Arc, Hamilton 1979, Wolfe 1983). Wolfe (1983) suggests that the Caramoan Peninsula and Catanduanes Island are the northernmost parts of this arc which have choked the trench which must have existed between the Talaud-Mindanao Arc and the western Philippines. Remnants of this trench may exist in the Lagonoy Gulf and the Albay Gulf, In view of this model, it seems quite possible that part of Camarines Norte also belongs to the envisaged accreted arc, an assumption that is strengthened by the already mentioned presence of tonalites in all three localities. The present paper presents the results of petrographic, geochemical, tectonic, and isotope studies of the NW part of the Paracale Intrusion, which is part of a broader investigation of the petrology and Sr isotope systematics of the Mid-Tertiary plutons of Luzon.

THE PARACALE INTRUSION is located on the northern shore of Camarines Norte, SE-Luzon (Fig. 1). It has an oval shape with a length of 17 km and a width of 4 km. A variety of dikes, which comprise aplites, pegrnatites, lamprophyres, mineralized quartz veins, and andesites cut through the intrusion. The pluton intruded ultramafic rocks which surround the entire intrusion and still overly it locally (Fig. 1). Alignment of biotite gives the intrusion the appearance of a WNW-ESE striking anticline the flanks of which dip outwards at about 40 °. The Paracale Intrusion is well known for its gold mineralization, which has been worked for several hundred years (e.g. von Drasche 1876, Smith 1910, Wisser 1939, Bryner 1969). The relationship of the trondhjemite intrusion to several other plutons in SE-Luzon is not clear and has been the subject of considerable speculation. Tonalite intrusions occur on the Caramoan Peninsula and on Catanduanes Island. They are believed to be of Lower Oligocene age and to be correlative with the Paracale REGIONAL GEOLOGY Intrusion (Miranda and Caleon 1979, and references cited therein). Age estimates for the Paracale Intrusion The geological make-up of Camarines Norte province range from Paleozoic to Pleistocene [Meek et al. 1941 (cited by Frost 1965 and Miranda and Caleon 1979), comprises rocks from the Mesozoic to recent. Most of Alvir 1950, Frost 1965, Miranda and Caleon 1979). A the formations present in the province crop out in the single K-Ar date of 14.9 Ma reported by Wolfe (1981) a r e a mapped for this study (Fig. 1). A series of chlodoes not resolve this question because the pluton shows ritized "eugeosynclinal" rocks of supposed Cretaceous age unconformably overlies a metamorphic basement of unknown age. The former series includes graywackes, * Present address: Department of Geology, University of Melbourne, Parkville, Victoria 3052, Australia. arkoses, mudstones, and subordinate spilites. It is un235

236

Uw~ GI~E et al.

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magmatic complex and the layered sequence formed in distant areas and have been brought into juxtaposition only by tectonic movements. The likelihood of such an evolution is increased by the proximity of the supposed northern tip of the Talaud-Mindanao Arc. In this case, the geologic history of both complexes may be completely different. The NW-SE trending fault zone, which separates the igneous complex and the sedimentary sequence, is about 1 km wide. Outcrops are lacking here because of intense weathering of the fractured rocks. At the SW-border of the fault zone, steep normal and reverse faults have been mapped. Only minor, curved faults dipping NE have been found at the NE margin. Here the ultramafics disappear below weathered material dipping about 30° SW (Fig. 1). THE ULTRAMAFIC COMPLEX

[]Foult zone [ ] Volconics

I

[ ] Universol formotion

[ ] Porocole intrusion

]Chloritized sediments

~ Ophiolite ~ complex

BB Metamorphic bosement

Fig. i. Geological map and schematic cross section o f the Jose Panganiban area. Inset shows the regional location o f this area; C = Caramoan Peninsula, CI = Catanduanes Island.

conformably overlain by a similar series, improperly known as "Universal Formation", which--in contrast to the underlying series--is not generally metamorphosed. Lithological comparisons indicate an Eocene age for the "Universal Formation" (Miranda and Caleon 1979). The "Universal Formation" is overlain by the andesitic "Larap Volcanics", which, however, are not exposed in the mapped area. The well known iron mine of Larap (Bryner 1969) is situated at the contact of the Universal Formation and the "Larap Volcanics" . Intrusive rocks of supposed Miocene age (Miranda and Caleon 1979) cut through the "Universal Formation" and the "Larap Volcanics", forming stocks, dikes, and necks. These rocks form a calc-alkaline series comprising diorites, microdiorites, andesites, dacites, and rhyolites (Giese 1983). The marls of the Universal Formation are locally metamorphosed to hornfelses at the contacts with diorite and microdiorite stocks. Iron skarns occur at the contacts of the intrusives. The relationship between the Paracale Intrusion and the rock series detailed above cannot be evaluated as the intrusion is hosted by a complex of ultramafic rocks which in turn is in fault contact with the layered sequences. The recognition that the ultramafic complex is an ophiolite (see below) which has been thrust upon the eastern shore of Luzon renders it possible that the

Good outcrops within the area occupied by the ultramafic complex are rare and found almost exclusively along the coast and along rivers. Thus little information could be obtained on the internal structure of the ultramafic complex. Harzburgite is the predominant rock type. It is usually serpentinized, being composed of antigorite and chrysotile with relics of olivine and pyroxene. An equigranular texture is sometimes still visible. The Al-serpentines antigorite and chrysotile are always present, their ratio, however, being variable. Often, antigorite is the main constituent of the ultramafics. It replaces olivine and orthopyroxene along fractures and grain margins. Where chrysotile is the main constituent, it exhibits "maschen"-structure. Bastite pseudomorphs after bronzite(?) occur. Actinolite, grammatite, chlorite, and dolomite and secondary alteration products. Chromite and secondary magnetite are accessories. In a triangular diagram showing weight percent MgO-CaO-AI203 (Coleman 1977), the chemical analyses of the ultramafic rocks (Table 1) plot in the field of metamorphic peridotites (Fig. 2). These rocks are considered to be depleted mantle materials (Coleman 1977), which differ from ultramafic rocks formed as cumulates by their lower contents of CaO and A1203. Within the field of metamorphic peridotites, the ultramafics from Camarines Norte follow a trend intermediate between the trends of the two suites observed in the Zambales range (Hock 1983). One of these latter suites is associated with refractory chromite ore and comprises peridotites, troctolites, and olivine gabbros, the other is associated with metallurgical chromite ore and comprises peridotites, pyroxenites, and noritic gabbros. Besides the harzburgites, one pyroxenite has been analysed (sample 204D). In the MgO-CaO-AI203 triangle (Coleman 1977), it plots in the field of ultramafic cumulates. Such ultramafic cumulates, as well as basalts, are found only in very subordinate amounts in the ultramafic complex of Camarines Norte.

The Paracale Intrusion

237

Table 1. Chemical composition of ultramafic rocks from the ophiolite complex. Analyses are calculated unhydrous Sample SiO2 (%) TiO 2 A1203 Fe203 MnO MgO CaO Ni (ppm) Co Cr V Cu

17 44.28 0.04 1.85 8.22 0.15 44.23 1.24 2286 47 2131 57 25

I1 45.39 0.05 1.84 9.57 0.17 42.35 0.62 2257 53 1941 64 42

9 44.17 0.02 1.04 9.16 0.13 44.32 1.14 2376 53 2351 56 33

100 44.42 0.02 1.05 8.80 0.15 44.82 0.71 2139 44 2294 46 28

4

15

43.78 0.03 1.40 8.20 0.13 45.49 1.03 2406 48 2319 49 9

45.12 0.03 1.49 9.66 0.16 42.01 1.53 2132 52 2829 81 46

204D

I 10B

45.27 0.14 4.71 10.26 0.09 35.72 3.80 1898 46 5091 103 22

49.38 I. 19 17.17 10.43 0.15 8.89 5.59 607 37 211 196 125

Total iron as Fe203. All samples are harzburgites except for sample 204D, which is a pyroxenite, and 110B, which is a tholeiitic basalt Na20 = 1.25%, K20 = 1.39%, P205 = 0.34% Rb = 27 ppm, Sr = 720 ppm, Ba -- 733 ppm.

Basalts occur as pillow-like bodies in black shales. They are altered rocks with a subophitic texture of plagioclase, clinopyroxene, and olivine. The chemical analysis of one sample (110B, Table 1) shows that it is high in K20 compared to common mid-ocean ridge basalts (MORB). This anomalous potassium content may, however, be of secondary origin. Other remarkable features are the high MgO/(MgO + FeO) atomic ratio of 0.67 (FeO = 0.85 x FeO t°t) and the high Ni content of 607 ppm. Both features suggest that the melt suffered little fractionation after separation from its mantle source. The presence of depleted mantle material, ultramafic cumulates, and basalts indicate that the ultramafic complex is an incomplete ophiolite complex. The upper parts of the ophiolite may have been eroded. However, a tectonic separation of the metamorphic peridotites and the cumulates during the emplacement of the ophiolite is also possible, because of the different rheology of these units (Coleman 1977).

THE PARACALE INTRUSION

morphic crystals with complex twinning and zoning. Sometimes an outer rim of albite surrounds these crystals. Albite forms anhedral, untwinned porphyroblasts. Quartz is subhedral and always shows undulatory extinction. Its grain size decreases with increasing deformation and recrystallization. The amount of biotite is variable and may be as high as 20 vol.%. The olive-green to dark brown crystals are always highly deformed. Alteration to chlorite is frequent. K-feldspar is not always present. Where it occurs it forms xenomorphic crystals with perthitic cxsolution and myrmekitic intergrowths at the margins. Accessory minerals are epidote, apatite, spheric, rutile, zircon and opaques. Calcite and sericite occur as secondary alteration products.

Geochemistry Three samples of coarse grained trondhjemite analyzed for major and trace elements (Table 2) show almost identical compositions with about 71% SiO2 and 17.5-18.3% A1203. The high AI content combined with the relatively low Ca, Na, and K concentrations results

Petrography The majority of the intrusive rocks comprising the Paracale Intrusion are coarse grained, leucocratic rocks with a distinct foliation. Fine grained or dark varieties are subordinate. Common constituents of these rocks are plagioclase (38-65 vol.%), quartz (32-42vo1.%), biotite (2-19 vol.%), and K-feldspar (1-2 vol.%). Dark varieties, which show enrichment of biotite, apatite, sphene, epidote, and opaques occur as xenoliths and schlierens. The mineralogical composition and the generally low colour index of the leucocratic varieties justify the use of the term trondhjemite (Barker 1979, Divis 1980). In thin-section, a strong tectonic overprint resulting in cataclasis and recrystallization is always visible. The fine grained, leucocratic rocks show the highest degree of deformation. Plagioclase occurs in two varieties. Oligoclase/ Andesine (An25_32) forms idiomorphic to hypidioS.E.A.E.S. I/4--D

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UWE GmSE et ai.

238

Table 2. Chemical composition of intrusive rocks from the Paracale Intrusion Sample SiO2 (%) TiO 2 A1203 Fe203 MnO MgO CaO Na20 K20 P2Os LOI Rb (ppm) Sr Ba Pb Zn Cu Zr

PG 825 70.1 0.1 18.4 1.6 0.02 4.3 4.2 1.3 0.03 0.0

x 71.0 0.2 18.3 1.5 0.03 0.7 3.3 4.3 1.6 0.05 0.8 17 1857 422 21 37 51 99

62c

x2

71.3 0.2 17.5 1.2 0.04 0.3 3.0 4.4 1.6 0.04 0.7 9 1647 315 25

97

71.6 0.2 17.8 1.7 0. I 0.5 3.2 4.2 1.8 0.05 1.0 29 1705 531 20 5 196 I 13

12

GW

67.6 0.6 14.7 4.9 0.1 1.7 2.3 3.2 2.3 0.27 0.8 42 1243 828 17 94 94 159

67.1 0.6 14.4 5.9 0.1 2.1 2.3 3.2 1.9 0.2 2.3

Total iron as Fe:O3. Sample 97 is a biotite-rich marie rock. PG 825: Anhydrous melt composition of a tholeiite melted at 825 C (Helz, 1976). GW: average composition of graywacke (Wedepohl 1967).

in a normative corundum content of 3-3.5 wr.% (CIPW norm), i.e. the rocks are peraluminous. Discrimination diagrams demonstrate affinities to the calc-alkaline magma series (Fig. 3). All major element abundances are well within the ranges considered to be typical for trondhjemites (Fig. 4, Barker 1979, viz. F e O t ° t + M g O < 3 . 4 % , CaO 1.5-4.5%, Na20 4.0-5.5%, K 2 0 < 2 . 0 % ) . The normative corundum content and the calc-alkaline affinities also are typical features of trondhjemitic suites (Barker 1979). Sample 97 represents the dark, biotite-rich variety of the intrusive rocks. The formation of these inclusions may be the result of assimilation of country rock. This assumption is strengthened by the observation that sample 97 has a composition that matches closely that of graywacke (Table 2). On the other hand, the high biotite content also may be due to enrichment of comagmatic biotite by some kind of flow differentiation.

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Sr-isotopic composition In an attempt to obtain more precise age data for the intrusion and in order to gain some insight into the origin of the magma, a Rb/Sr isotope study was undertaken. The results are listed in Table 3. As all the trondhjemites have very low Rb and high Sr, these rocks show almost no spread with regard to their STRb/SrSr ratios, which is required for the construction of an isochron. Biotite separated from the trondhjemite also turned out to contain a high amount of Sr, a common feature in Sr-rich rocks (see data of Knittel 1983). Thus no reliable mineral isochron could be constructed. Figure 5 shows that the isotope data are compatible with any age between 0 and 14.4 Ma for the most recent "thermal event" which may be the final cooling of the pluton or a metamorphic event. The STSr/86Sr ratio determined for the coarse trondhjemite (0.70352) is considered to represent the initial

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239

The Paracale Intrusion Table 3. Results of the mass spectrometric study of samples from the Paracale Intrusion Sample

Material

Rb (ppm) Sr (ppm) STRb/~Sr

X X 43

wr total. biotite wr biot. inclusion RSA 1 wr greensehist

STSr/~Sr

16 131 100

1901 255 653

0.024 1.49 0.44

0.70352 + 0.00011 0.70366 + 0.00005 0.70353 + 0.00005

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0.70316 + 0.00006

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equilibrium was reached during the reaction with the melt or that the country rock had about the same isotopic composition as the melt. Tectonics

As already noted, the intrusives show a distinct foliation produced by biotite alignment (Fig. 6). The strike is consistently NW-SE while the dip changes from a SW direction at the SW margin of the intrusion to steep NE at the northeastern margin of the exposure. The foliation thus is anticlinal in form. Measurements of the orientation of the biotite flakes in thin-section confirm the observation of Bryner (1963) that the trondhjemite is a B-tectonite (Figs 7 and 8). The quartz crystals do not show the girdles of biotite but simple alignment. At the SW margin of the intrusion, a broad zone No20 K20 about 100 m wide is marked by very strong tectonic Fig. 4. C a O - K 2 0 - N a 2 0 plot for the Paracale trondhjemites (circles). overprint giving the trondhjemites a gneissic appearance. Trondhjemites from Canyon Mountain Ophiolite (crosses, C-erlach et Similar rocks are found within the intrusion forming al. 1981) and Hijaz (triangle, Jackson et al. 1984) are shown for comparison. Also shown are melt compositions obtained by melting of "dikes" of porphyritic appearance. In thin-section, a fine tholeiites at 5 kbar water pressure (see Fig. 3). grained matrix of biotite, quartz, and feldspar can be seen. Large "phenocrysts" of plagioclase and K-feldspar are fragments of formerly larger crystals. The foliation Sr isotopic composition because of the low Rb/Sr ratio. is created by the alignment of biotite and recrystallized It is identical with the ratio obtained for the biotite-rich quartz-ribbons. These rocks are best explained as myinclusion, but is slightly higher than the value of 0.70323 lonites which originated in response to tectonic stress reported by Divis (1983), who, however, did not provide during the final stages of consolidation of the melt after any information on the normalization procedures used emplacement (Waters and Krauskopf 1941). Jointing in the trondhjemite complex follows two in this study. If the biotite-inclusion is considered as assimilated country rock, this implies that isotopic directions: The most prominent one strikes N-S and

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UWE GIESE et al.

240

Aplites and pegmatites Numerous aplitic dikes cut through the intrusion. They continue into the ultramafics where they can be traced for several 100 m. The aplites show a fine grained texture with phenocrysts of feldspar ( - 1 cm) and garnet (1 mm). The composition of these rocks is similar to that of the trondhjemite except that the mafic constituents are almost totally lacking. Quartz, plagioclase, and Kfeldspar are the main constituents. Garnet (almandinerich) forms corroded phenocrysts. 17~

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Fig. 6. Equal area pole projection (lowerhemisphere)of the schistosity of the Paracale Intrusion (64 poles, maximum 210/39).

dips E, while the other one strikes N E - S W (Fig. 9). The first direction is well developed in the western part of the mapped area. In the eastern part it is rotated to the N E - S W direction. The mineralized veins follow this pattern, i.e. they strike N - S in the west and N E - S W in the east. A strong tectonic overprint to which the ultramafics have been subjected is seen in Fig. 10, which summarizes the joint directions of this complex. A few measurements were also taken in the "Universal Formation" in which joint directions are similar to those observed in the trondhjemite complex (Fig. 11).

DIKES CUTTING T H R O U G H THE C O M P L E X

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Nearly all late intrusive phases such as aplites and pegmatites strike NNE-SSW. This is especially true for the lamprophyres and the quartz veins but can be observed also for the andesite dikes. Aplites, quartz veins, and andesitic dikes extend into the ultramafic complex providing evidence that the present spatial relationship between the two complexes at the time of dike emplacement was the same as today. Aplites and pegmatites show a strong tectonic overprint while lamprophyres are not deformed. The former thus may be contemporaneous with the intrusion while the lamprophyres are obviously younger. Chemical analyses of dike rocks are listed in Table 4.

b-lineation

-I,75 -3,35 -55 -6,75

Fig. 7. Idealizedsketch of the Paracale trondbjemite to illustrate the foliationproduced by the alignmentof biotite and the deformationof the biotite flakes.

(b) Fig. 8. (a) Poles to (100) of biotite (1(~ measurements, equal area projection, lower hemisphere). (b) Poles to quartz axes (60 measurements, equal area projection, lower hemisphere).

The Paracale Intrusion

241

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Two types of pegmatites occur as thin, discontinuous dikes. One is composed predominantly of coarse quartz and feldspar while the other is a biotite pegmatite with biotite flakes of up to 10 cm in length.

Lamprophyres Lamprophyres are rare and have been discovered only along the NE margin of the intrusion. They form discontinuous and irregularly shaped dikes up to 2 m wide. Besides macrophyric plagioclase, in thin-section phenocrysts of biotite, amphibole, and quartz are seen. The feldspar is completely replaced by calcite. It may be identified as formerly zoned plagioclase on the basis of

20

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its texture. Biotite crystals, in contrast, look fresh. The amphibole is a barkevikitic hornblende, typical of lamprophyres. Quartz crystals, which always show strong corrosion, are rare. They may be xenocrysts. The matrix of the lamprophyres is subophitic to intergranular. It consists of plagioclase, biotite (pseudomorphous after amphibole), amphibole and ?quartz. On the basis of its mineralogy, this rock may be classified as kersantite (Turner and Verhoogen 1960, Streekeisen 1980).

Andesites Several dikes of andesitic composition strike N - S in the ultramafic complex near Gumaus (Fig. 1). The

242

Uw~ GI~E et al. Table 4. Chemical analyses of dike rocks and a xenolith from the Paracale Intrusion Sample Rock SiO2 (%) TiO 2 A1203 Fe203 MnO MgO CaO Na20 K20 P205 Rb (ppm) Sr Ba Ni Cr

51A pegmatite 76.5 0.2 15.1 0.5 0.02 0.0 1.2 2.9 4.3 0.0 22 770 1166

114A aplite 77.3 0.1 14.6 0.3 0.1 0.0 1.0 2.9 4.0 0.0 28 299 231

66/2 lamproph,

48/1 andes

48/2 andes

RSAI greenshist

49.1 0.6 12.7 6.7 0.1 7.7 7.8 0.7 2.6 0.4 13 9395 1409 177 310

61.7 0.4 15.1 4.2 0.2 3.2 4.3 0.0 4.6 0.1 100 127 181

61.1 0.4 15.1 4.3 0.2 2.2 4.3 0.0 4.6 0.1 98 109 187

52.1 0.6 14.9 8.7 0.2 5. I 7.7 3.0 3.6 0.9 46 2956 n.d.

andesite is a grey coloured, porphyritic rock. PhenoGold is associated with nearly all quartz types but is crysts are euhedral, zoned plagioclase crystals up to most abundant in white quartz. Here, pyrite is the most 5 mm in size, commonly replaced by sericite, calcite, and prominent host of gold. It is, however, also found as sulfides. Euhedral pyrite grains are found disseminated inclusions in sphalerite and galena and in small veins in in the matrix or in tiny veins. Quartz and apatite are cataclastically fractured pyrite. The gold has an intense present in subordinate amounts. The groundmass is yellow colour indicating a low Ag content. composed of sericite, calcite, and quartz. As the trondhjemite and its late intrusive phases do In polished-sections pyrite shows idioblastic growth. not show elevated metal contents, while the andesites are It contains numerous inclusions of silicates, sphalerite strongly altered and mineralized, it is thought that and gold. In addition to pyrite, sphalerite with variable andesites and quartz veins are genetically related. iron content is observed. Galena and chalcopyrite are found in small quantities. The andesitic dikes are strongly altered as exemplified XENOLITHS by the alteration of feldspar and by the replacement of the mafic minerals by sulfides. The different kinds of xenoliths have been found in the Because of the lack of mafic minerals these rocks are Paracale Intrusion. Ultramafic rocks forming a remnant difficult to classify. The chemical analyses support the of the roof of the intrusion (?) may be mapped at the assumption of an intermediate composition. However, scale of the map (Fig. 1). These ultramafic rocks are they show strong potassium enrichment, while sodium altered to assemblages of grammatite, actinolite, and probably has been leached. The high degree of alteration subordinate amounts of plagioclase and quartz. A disof the studied samples prohibits evaluation of possible tinct saussuritisation is always visible. relationships between these rocks and the young volcaThe second type of xenoliths comprises greenshists nic rocks which outcrop SW of the intrusion. with a granoblastic texture containing biotite, albite, and quartz. Two samples of these rocks have been recovered Quartz veins from drill cores sunk in the Santa Rosa Mine area (at depths of 37 m and 108 m). The quartz veins generally strike NE/NNE and can be In one sample, relics of amphibole, epidote, and traced for several kilometres. They vary in thickness chlorite have been observed in the matrix. This may from less than 1 cm to about 7 m. indicate that the greenshists experienced two metamorThe association of quartz veins with the dikes is phic events. The original composition and nature of this ascribed to common utilization of the same main joint rock are difficult to establish; its chemical composition direction rather than any genetic relationship. is given in Table 4. From its CIPW-norm it may b e The quartz varies in composition and appearance. concluded that these greenshists are derived from basic Green quartz containing minute sericite flakes formed at latites. Their comparatively low silica and aluminum the contacts with the wall rocks. White quartz is associ- contents makes a sedimentary origin unlikely. The very ated with galena and sphalerite. Glassy, clear quartz low 87Sr/S6Sr ratio of 0.7032 of the analyzed sample occurs associated with copper minerals while layered combined with its very high Sr content ( - 3000 ppm) is quartz crystals containing pyrite were deposited later. puzzling. In view of the low Rb/Sr ratio the Sr isotopic These quartz varieties do not represent several phases of ratio certainly represents the initial Sr isotopic comhydrothermal activity, but are considered to reflect position. Alternatively the Sr may have been introduced changing physico-chemical parameters on the hydro- into this rock. The low STSr/S6Srratio lies at the upper thermal fluids. margin of the values commonly observed for fresh or

The Paracale Intrusion slightly altered MORB indicating a possible origin from oceanic basalts undergoing metamorphism. The contact between the greenshists and the trondhjemite host rocks in both core samples is tectonic, which points to a possible transport of the xenoliths along faults into the intrusion. Bryner (1969) likewise reported the occurrence of ultramafics at several levels of the Paracale Gumaus Mine forming a downfaulted, serpentinized wedge. DISCUSSION Trondhjemites are known to occur in three Cenozoic environments, orogenic continental margins, island arcs, and ophiolites (Barker 1979). The setting of the Paracale Intrusion in an island arc precludes an origin at a continental margin, while the close spatial relationship to an ophiolite sequence requires some consideration of the third possibility. Trondhjemitic plagiogranites, the leucocratic members of ophiolite sequences differ chemically from the trondhjemites of the Paracale Intrusion by lower A1202 (<16%) and K20 contents ( < 1 % ) (Coleman 1977, Gerlach et al. 1981). The initial Sr isotopic composition of trondhjemites (87Sr/SrSr=0.7035) likewise argues against a genetic relation between the intrusives and the ophiolite sequence as this value is distinctly higher than the values commonly observed for MORB. This evidence is however not unequivocal as samples from ophiolites yield 87Sr/arsr ratios as high as 0.7060 because of the introduction of radiogenic Sr from seawater (Coleman 1977). The intrusive relationship of the trondhjemites to the ultramafics also does not argue conclusively against a genetic relation between these units but renders it unlikely as the plagiogranites usually are found in the upper part of the ophiolite sequence and not associated with the metamorphic ultramafics (Coleman 1977, Gerlach et al. 1981). The occurrence of trondhjemites in island arcs is considered to be the consequence of partial melting of amphibolitic oceanic or chemically primitive island arc crust under high water pressures and high thermal gradients (Gill and Stork 1979, Jackson et al. 1984). Experimental data obtained by Helz (1976) show that melting of tholeiitic starting materials at PM2o= 5 kb and temperatures of 825-875°C produces quartz-rich, corundum-normative melts closely approximating the composition of the Paracale trondhjemites (Table 2 and Fig. 4). Melting at lower temperatures produces melts richer in silica (>74%) while melting at higher temperatures produces melts richer in AI compared to the levels observed for Paracale. The starting material used in these experiments differs from common island arc tholeiites only by its low A1203 content (14.5% compared to 18-19% in island arc tholeiites, Whitford et al. 1979). The experimentally produced and the observed compositions thus show a surprisingly good correspondence except for MgO. MgO contents as low as those in the experimentally produced liquids, however, are rarely

243

found in natural rocks (Coleman 1977, Gerlach et ai. 1981). The interpretation of the trondhjemites as the product of partial melting of amphibolite could provide an explanation for the quite uniform composition of the intrusives, which provide no clue to their being the final products of prolonged differentiation. The low Sr isotopic ratio observed for the trondhjemites is compatible with this interpretation, because the amphibolite source may be considered to be derived from basalts which generally have low Sr isotopic ratios and sufficiently low Rb/Sr ratios to preserve the low Sr isotopic ratio for long times. The high Sr content of the Paracale trondhjemites is a somewhat puzzling feature. It is about two to three times higher than the values commonly observed for trondhjemitic rocks (Peterman 1979). The study of Helz (1976) has shown that tholeiites at Pr~2o= 5 kbar are composed mainly of amphibole and plagioclase. At 825°C, 20-35% of the rock is molten, the melt being mainly formed from plagioclase. Because most of the Sr present in the amphibolite is incorporated in plagioclase, the melt could be enriched in Sr by a factor of about three over the starting material. This implies that the source of the Paracale melts had a Sr content of about 500-600 ppm, twice as much as is commonly observed in island arc tholeiites (Whitford et ai. 1979). The Sr content of the melt may, however, have been increased by accumulation of plagioclase. Evidence for this assumption is seen in the coherent behavior of CaO, A1203, and Sr (Fig. 12), which all are components of the anorthite component of plagioclase. The mechanism has also been proposed for trondhjemites associated with the Amitsoq gneisses in southern West Greenland by Nutman et al. (1984) to explain Sr enrichment and depletion in Rb and Ba. The lack of cumulus textures in the Paracale and Amitsoq trondhjemites may be explained by the strong tectonic deformation of the intrusives. The suggested origin of the intrusives as cumulates also would provide an explanation for the strong tectonic deformation of the trondhjemites during the final emplacement, because rocks containing high amounts of crystalline phases take on many properties of elastic solids (McBirney and Noyes 1979). The alignment of the biotite crystals represents the simultaneous development of flow cleavage and foliation.

CaO

Sr

[O/o]

[ppml I

I

I

I

I

I

I

I

I _

1900

3.4

sioa''~ .22 ° /

-

_

3.2

3.0

~

I 17.6

I

I 17.8

I

I 18.0

I

_

1800

1"/00

I 18.2

I [%]

A[203

Fig. 12. CaO and Sr vs Al~O3 for trondhjemitcs of the Paracale Intrusion.

244

UwE GI~E et al.

Further evidence for the model proposed is seen in the lack of a major contact aureole pointing to relatively low temperatures at the time of intrusion. This is compatible with the assumption of a flotation cumulate close to its solidus temperature. Compressive stress during or after emplacement resuited in the formation of cross and oblique joints. These joints have been reactivated repeatedly during later deformations and served as pathways for the andesites, quartz-veins, and lamprophyres. In thin-section later deformation is recognized as advanced cataclasis, shortening of the biotite flakes and blastesis of albite. The relative sequence of events may be inferred from the observed field relations. The emplacement of the intrusion was followed by the intrusion of aplites and pegmatites. Subsequently the complex was subjected to tectonic stress resulting in the formation of mylonites, cataclasis and recrystallization. This tectonic event may be the result of west-directed overthrusting of the whole magmatic complex (ophiolite and trondhjemite) onto Luzon. The age of the Paracale Intrusion is still uncertain as no whole-rock isochron could be established. The K-Ar age of 14.9 Ma (Wolfe 1981) indicates a Mid Miocene age. The Sr-isotopic data are compatible with an "event" of that age. However, this event may also be a tectonic overprint or the date obtained is a meaningless figure resulting from incomplete rejuvenation of the isotope systems. Miranda and Caleon (1979) regard the diorites, microdiorites and related rocks of the Larap Peninsular as being of Mid Miocene age. As indicated before, the emplacement of the andesite dikes and the mineralized quartz veins seems to be contemporaneous with the intrusion of these rocks. Thus, the Paracale Intrusion must be older. Because of structural considerations and comparisons with other intrusions in SE-Luzon (Caramoan, Catanduanes), Miranda and Caleon (1979) assume an early Oligocene age for the Paracale Intrusion. The petrographic descriptions of these intrusives, however, do not show clear similarities with the Paracale Intrusion and chemical data are not yet available. Despite this, their age estimate at present seems to be the best available one. It would place the Paracale Intrusion with regard to age with the large plutons of the Sierra Madre and the Caraballo Mountains of North Luzon, the intrusions of Polillo Island (Knittel and Kramm, in print) and possibly also the Agno Batholith of the North Luzon Cordillera Central (Balce et al. 1980).

Acknowledgements--UG wishes to express his gratitude to Billiton Philippines Inc. and to Dr C. K. Burton for financial and logistic support during the field work and to Bong Polente for his company in the field. R. v. Hoegen kindly spent time to discuss with us the tectonics of the area. W. Winter helped process the tectonic data with the aid of a computer, H. G. Brunemann provided the word-processing facilities, which is gratefully acknowledged. Careful reviews by Dr C. K. Burton and Dr M. Defant helped to improve the manuscript.

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