Mio-Pliocene magmatism in the Baguio Mining District (Luzon, Philippines): age clues to its geodynamic setting

C. R. Acad. Sci. Paris, Sciences de la Terre et des planètes / Earth and Planetary Sciences 331 (2000) 295–302 © 2000 Académie des sciences / Éditions scientifiques et médicales Elsevier SAS. Tous droits réservés S1251805000014154/FLA

Géodynamique/ Geodynamics

Mio-Pliocene magmatism in the Baguio Mining District (Luzon, Philippines): age clues to its geodynamic setting Hervé Bellona*, Graciano P. Yumul Jr.b a UMR 6538 « Domaines océaniques », IUEM, université de Bretagne occidentale, av. Victor-Le-Gorgeu, BP 809, 29285 Brest cedex, France b Rushurgent Working Group, National Institute of Geological Sciences, College of Science, University of the Philippines, Diliman, Quezon City, Philippines

Received 3 January 2000; accepted 10 July 2000 Communicated by Jean Aubouin

Abstract – The Baguio Mining District (Central Cordillera of Luzon Island) has evolved, from Eocene to Recent time, from a marginal basin to an island arc setting, related firstly to a westward subduction and after a subduction polarity reversal by the Early Miocene to an eastward subduction. Late Miocene to Quaternary magmatism and tectonics have allowed the deposition of gold and copper. This scenario is consistent with available geological field and isotopic K–Ar ages. © 2000 Académie des sciences / Éditions scientifiques et médicales Elsevier SAS K–Ar ages / metamorphism / island arc magmatism / Cu–Au mineralization / Baguio / Philippines

Résumé – Chronologie du magmatisme mio-pliocène du district minier de Baguio (Luzon, Philippines) : contraintes temporelles du contexte géodynamique. Le contexte géodynamique du district minier de Baguio (cordillère centrale de Luzon) a évolué au cours du temps, depuis un bassin marginal jusqu’à un arc insulaire. Les multiples événements magmatiques enregistrés sont reliés à des subductions, originellement à vergence ouest, puis à vergence est après le Miocène inférieur, à la suite d’un renversement de la polarité des subductions. © 2000 Académie des sciences / Éditions scientifiques et médicales Elsevier SAS âges K–Ar / métamorphisme / magmatisme d’arc insulaire / minéralisation Cu–Au / Baguio / Philippines

Version abrégée 1. Introduction Le district minier à or et cuivre de Baguio (île de Luzon, Philippines) se situe dans le complexe volcanoplutonique d’arc de la cordillère centrale de Luzon, à 1 500 m d’altitude. La minéralisation aurifère vient clore, au Pliocène supérieur, une histoire géodynamique, magmatique et tectonique complexe [1, 6], qui a débuté au cours de l’Éocène. La formation métavolcanique de Pugo (pillow lavas, coulées et brèches basaltiques à andésitiques), qui

constitue le substratum de l’arc, est recouverte en discordance par la formation Zig-Zag, conglomératique et riche en fragments volcaniques, comparables par leur géochimie aux laves de la formation Pugo. La formation Zig-Zag est recouverte par les calcaires récifaux de Kennon, datés « N7–base de N8 » et dépourvus de toute roche magmatique, à leur tour recouverts en discordance par la formation Klondyke, déposée après le soulèvement et l’érosion de la cordillère centrale plutonique. Le changement observé dans la composition des fragments magmatiques présents dans les deux forma-

* Correspondence and reprints: [email protected]

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H. Bellon, G. P. Yumul Jr. / C. R. Acad. Sci. Paris, Sciences de la Terre et des planètes / Earth and Planetary Sciences 331 (2000) 295–302

tions discordantes montre le passage d’un contexte de bassin marginal à un contexte d’arc insulaire. 2. Résultats principaux 2.1. Âges isotopiques des manifestations magmatiques Les âges 40K–40Ar, mesurés sur roche totale, des différents corps et unités magmatiques de trois districts miniers sont les suivants (tableau): dans le district de Philex, 18 Ma sur une andésite de Pugo, 15,5 et 13,1 Ma pour les diorites d’Ansagan et de Tailings Dam, enfin 3,70 à 2,30 Ma sur les différents faciès du corps minéralisé de Philex ; dans le district d’Antamok, de 19,9 et 14,7 Ma pour les schistes de Dalupirip et 13,4 et 11,9 Ma pour les granodiorites et diorites ; dans le district de Santo Nino, un pluton de granodiorite est daté à 16,9 Ma. 2.2. Caractéristiques géochimiques des manifestations magmatiques Les spectres multi-élémentaires (figure 2) des andésites de la formation Pugo et des schistes de Dalupirip sont très semblables et sont transitionnels entre ceux de MORB (net enrichissement de Rb à Sm) et ceux de laves d’arc (anomalies négatives en Nb et Zr). On peut raisonnablement penser que les laves de Pugo, typiques d’un magmatisme d’arrière-arc, constituent le protolithe de ces schistes. Le magmatisme pliocène (52 < SiO2< 62 %) présente les signatures typiques des magmas d’arc ; les laves les plus récentes se caractérisent, en outre, par des teneurs basses en terres rares lourdes (Yb = 1,1 ppm), des rapports Sr/Y de 35 et des concentrations en Y de 9 ppm, caractéristiques des magmas adakitiques. Une transition des magmas calco-alcalins aux magmas adakitiques est donc observée. Temporellement, les magmas adakitiques précèdent immédiatement la minéralisation aurifère. Un lien spatio-temporel entre les minéralisations aurifères et le magmatisme adakitique est donc à nouveau constaté et vient confirmer la liste dressée par Sajona et Maury [14].

1. Introduction The Baguio Mining District is part of the Central Cordillera magmatic arc of Northern Luzon which exemplifies the complex volcano-plutonic and geodynamic evolution that the Philippine island arc system has undergone. This major gold–copper producing area has attracted numerous studies because of its complete assemblage of marginal basin to island arc-related igneous, sedimentary and metamorphic series, e.g. [1, 6]. We present here fifteen new 40K–40Ar whole-rock ages of volcanic, intrusive and metamorphic rocks collected

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3. Discussion Les âges isotopiques et la composition chimique des schistes de Dalupirip conduisent à interpréter cette formation comme un faciès particulier de la formation métavolcanique Pugo métamorphisée au contact des plutons sous-jacents de la Cordillère centrale. Le district de Baguio a été le siège, au moins depuis le Miocène moyen, d’une intense activité magmatique, attestée par les 15 âges 40K–40Ar sur roche totale de roches plutoniques, pour les témoins les plus anciens, et de roches volcaniques et plutoniques, pour les témoins plus récents. Ces âges sont présentés dans le tableau et comparés aux résultats antérieurs [13], abondés de quelques données sur minéraux séparés, dans le diagramme de la figure 3. Pour un même échantillon, la confrontation des âges mesurés sur roche totale et sur deux fractions minérales séparées (amphiboles et feldspaths) met en évidence l’âge plus ancien des amphiboles, l’âge plus récent des feldspaths et, enfin, l’âge intermédiaire, mixte, de la roche totale. Ce résultat reflète les proportions modales de ces minéraux dans la roche. En tenant compte des températures de fermeture des chronomètres isotopiques, différentes, il est suggéré que les amphiboles conservent l’âge initial, celui de la mise en place, et que les feldspaths ont subi une réouverture partielle, voire totale, lors de la mise en place des nouveaux magmas. Au total, les magmas observés dans le district de Baguio se seraient mis en place entre 23 et 2 Ma. L’augmentation de la teneur en potassium dans les magmas est notable (figure 4) et se manifeste au moins à trois reprises vers 14–13, 7 et 3 Ma. L’évolution géodynamique du secteur septentrional de l’île de Luzon est marquée par le renversement de la polarité de la subduction, à vergence ouest à l’Oligocène, et à vergence est (subduction manillaise) au cours du Miocène. L’examen comparatif des données chronologiques et l’interprétation des variations magmatiques permettent de proposer que ce renversement, ainsi que le début de la subduction de la jeune mer de Chine méridionale, ont pris place entre 19 et 17 Ma, au cours du Burdigalien supérieur.

from the mining district during a fieldwork we conducted in October 1997. These data together with the previous data [13] give an insight on how this particular part of the Philippine island arc system had evolved through time.

2. Geological setting Northern Luzon island is bounded on the west by the east-dipping Manila Trench and on the east by the westdipping East Luzon Trough–Philippine Trench (figure 1).

H. Bellon, G. P. Yumul Jr. / C. R. Acad. Sci. Paris, Sciences de la Terre et des planètes / Earth and Planetary Sciences 331 (2000) 295–302

120°35'

D

120°40'

HR6

Quaternary (?) Dikes

D

Sto Nino

Mirador limestone

D

Pico formation Klondyke Formation Kennon Limestone

Qual.

Zigzag Formation

Am

bu k

Pugo Metavolcanics

lao

BAGUIO CITY

Central Cordillera Intrusive Complex Black Mountain Intrusive Complex

16°20'

Ro

ad

Dalupirip Schist

Antamok

Road

AR3 Keystone AR2 a et b

Inferred Faults

AR2

Sample Location Mine

Black Mt

AR4 Thanksgiving Acupan PhR6 ilex

Ph

Itogon Suyon mine

Ro ad

N 5 km Scale 121°

16°15'

LUZON

M

anila Tr en c h

East Luzon Tr ou g h

STUDY AREA

BAGUIO CITY

Figure 1. Carte géologique du secteur minier de Baguio, montrant la localisation des échantillons datés et listés dans le tableau.

125°

Clt 18°

Figure 1. Geological map of the Baguio sector and location of the dated samples listed in the table.

PhM2b,M5,M6a Sto. Tomas II (Philex)

TD

ST

N 100km

14°

The Philippine Fault, a major transcurrent sinistral fault, cuts the Philippine archipelago along its whole length, and separates into several splays in Northern Luzon, e.g. [11, 12]. The Baguio Mining District, located about 1 500 m above sea level in the Luzon Central Cordillera magmatic arc (figure 1), is floored by the Pugo Metavolcanics Formation, generated in a subduction-related mar-

PhR2 PhR1

An2 Ansagan Philippine Fault Zone

ginal basin, and consisting of pillow basalts and andesites, basaltic andesite flows and breccias. Along the Ambuklao Road, it grades into the Dalupirip Schist. Various outcrops of this formation expose hornfels, metamorphosed andesites, amphibolites, chloriteepidote schists, and tuffaceous volcanic-sedimentary rocks. Preliminary rock analyses show its geochemical similarity to that of the Pugo Metavolcanics [22]. The

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H. Bellon, G. P. Yumul Jr. / C. R. Acad. Sci. Paris, Sciences de la Terre et des planètes / Earth and Planetary Sciences 331 (2000) 295–302

Table. 40K–40Ar whole-rock ages. Tableau. Âges 40K–40Ar sur roches totales.

Sample code

Nature and location

Philex mine district Clt 1 granodiorite Philex (Clifton) Ph-M6a andesite porphyry Philex mine

Mean age (My)

Lab. Ref.

Age ± error (My)

K2O (wt%)

2.33 ± 0.23

B4791 B4817 B4640 B4819 B4654 B4647 B4643 B4659 B4642 B4651 B4653 B4820 B4639 B4638 B4652 B4664 B4790 B4786 B4785 B4789 B4794 B4788

2.29 ± 0.23 2.38 ± 0.18 2.25 ± 0.21 2.26 ± 0.19 2.42 ± 0.23 2.29 ± 0.22 2.43 ± 0.20 2.63 ± 0.25 2.39 ± 0.10 2.50 ± 0.11 2.89 ± 0.26 3.08 ± 0.21 3.12 ± 0.26 3.31 ± 0.78 3.30 ± 0.68 3.64 ± 0.50 3.68 ± 0.35 3.74 ± 0.39 12.87 ± 0.32 13.34 ± 0.33 15.52 ± 0.45 15.51 ± 0.48

0.75

B4818 B4781 B4779 B4783 B4663 B4708 B4821 B4641 B4665 B4778 B4782

11.74 ± 0.67 12.01 ± 0.71 13.37 ± 0.35 13.34 ± 0.34 14.61 ± 1.43 14.71 ± 1.64 14.89 ± 1.53 17.69 ± 2.00 18.43 ± 2.00 19.79 ± 1.23 19.90 ± 1.16

B4780 B4784

16.57 ± 0.41 16.96 ± 0.45

2.31 ± 0.21

Ph-R1

andesite Philex road

2.45 ± 0.25

Ph-R6

andesite Philex road diorite Philex mine

2.45 ± 0.11

Ph-M2b

dark diorite Philex mine

3.42 ± 0.78

ST 34 Q1

diorite Santo Tomas granodiorite Tailings Dam dark fine-grained diorite, Ansagan

3.71 ± 0.35

Ph-M5

TD AN2

Antamok mine district AR2a diorite Antamok road AR4 granodiorite Antamok road AR3 Dalupirip schist Antamok road Ph-R2 AR2b

meta-andesite Pugo fm, Philex road Dalupirip schist Antamok road

Santo Nino mine district HR6 dark diorite Halsema road

3.03 ± 0.26

13.1 ± 0.3 15.5 ± 0.5

11.9 ± 0.7 13.4 ± 0.3 14.7 ± 1.6 18.1 ± 2.0 19.9 ± 1.2

16.9 ± 0.4

Dalupirip Schist is intruded by highly deformed dioritic and trondjhemitic dykes and is cut by a probable Early Miocene strike-slip fault [10]. The Pugo Metavolcanics Formation is unconformably overlain by the Zigzag Formation, made up of interbedded green sandstones, red siltstones, oligomictic conglomerates and minor limestone units. A biomicrite from the Malaya River yielded Eulepidina ephippiodes, Nummulites sp. and Heterostegina sp., all indicating an Early Oligocene age; another biomicrite yielded Lepidocyclina sp., Eulipidina sp., Operculina sp. and Meolobesiaes algae indicative of Early Oligocene to very Early Miocene age [7, 10]. Basaltic to andesitic clasts in the conglomeratic part are geochemically similar to the Pugo Metavolcanics rocks [21].

298

0.91

0.48

1.42 0.68

0.18

0.50 1.26 0.61

0.18 0.83 0.092

0.065 0.085

0.88

40 ArR (10–7 cm3·g–1)

40

ArR (%)

0.553 0.575 0.659 0.663 0.711 0.354 0.377 0.406 1.095 1.147 0.633 0.675 0.685 0.192 0.191 0.211 0.594 0.604 5.245 5.450 3.065 3.064

8.3 11.0 9.1 10.6 8.7 7.8 13.3 10.3 22.0 20.3 9.5 13.1 10.3 3.7 4.4 6.7 9.0 8.1 53.8 60.0 49.0 45.7

0.702 0.697 3.568 3.584 0.435 0.438 0.444 0.372 0.388 0.545 0.547

32.7 27.0 44.7 48.7 19.0 17.4 44.4 14.6 21.6 24.0 31.2

4.723 4.835

55.2 43.5

The late Early Miocene to early Middle Miocene Kennon reefal limestone (N7 to lower N8 [2] in Ringenbach [12]; Tf1 to Te5 [10]) overlies conformably the Zig-Zag Formation. This limestone unit is unconformably overlain by the early Middle to middle Late Miocene Klondyke Formation (calcareous nannofossil Zones NN5–NN10 [5] with a corresponding age of 16 to 8.5 My [4]) that contains clasts of diorites and gabbros, that were derived from the erosion of the plutonic Central Cordillera. The Middle Miocene to Pleistocene period was also marked by the emplacement of island arc-related dioritic to gabbroic plutons [23], and widespread multipulses volcanic calc-alkaline activity with adakitic lavas,

H. Bellon, G. P. Yumul Jr. / C. R. Acad. Sci. Paris, Sciences de la Terre et des planètes / Earth and Planetary Sciences 331 (2000) 295–302

1000.00 SiO 2 (wt%)

Rock / PM

100.00

PhM6a PhM5 PhR1 PhM2b PhR6

63.5 61.5 61.2 56.8 52.6

PhR2 AR3

55.2 53.0

10.00 Figure 2. Mantle-normalized [17] spidergrams for Pugo Metavolcanics and Dalupirip Schist (filled symbols) and Pliocene Baguio magmatism (open symbols). Figure 2. Spectres multiélémentaires normalisés au manteau primitif [17] des métavolcanites de Pugo et des schistes de Dalupirip (symboles pleins), ainsi que du magmatisme pliocène de Baguio (symboles évidés).

1.00

0.10

Rb Ba Th

supposedly generated during the partial melting of the subducted South China Sea crust [13]. The change in clast components in the conglomeratic units from the Zigzag to the Klondyke Formation signalled the change from a marginal basin (Pugo Metavolcanics) to an island arc setting for the Baguio Mining District.

3. Isotopic ages and geochemical features Fifteen whole-rock 40K–40Ar datings have been performed at the Geochronology Laboratory of the University of Bretagne occidentale using the methodology described in [3]. Samples are located on the geological map (figure 1) and calculated ages, using the constants of Steiger and Jäger [16] with the uncertainties along Mahood and Drake [9], are listed in the table for the mine areas of Philex, Antamok and Sto. Niño. Seven samples have been analysed for their major and trace elements by inductive coupled plasma. Figure 2 shows their spidergrams. 3.1. Ages

Two samples from Dalupirip Schist, collected near the Antamok Mines and at the junction of the Liang and Ambalanga Rivers yield mean ages of 19.9 My (AR2b, a xenolith in a diorite (AR2a) collected in Antamok complex) and 14.7 My (AR3), respectively. An andesitic sample (Ph-R2) from the Pugo Metavolcanics, in contact with a dioritic body as exposed along the Philex Mine road yields a mean age of 18.1 My. A

K Nb La Ce Sr

Nd Sm Zr Eu Gd Ti Dy

Y

Er Yb

gabbro sample (HR6) taken in the vicinity of the Sto. Ninˇ o Mine along Halsema highway gives an age of 16.9 My. Whole-rock dioritic samples from Ansagan and Agno Batholith outcrop at the Philex Mine Tailings Dam are dated at 15.5 My (AN2) and 13.1 My (TD), respectively. A diorite sample (AR4) from Itogon is dated at 13.4 My. Lastly the xenolith-bearing Antamok diorite (AR2a) is dated at 11.9 My. The new data for the Philex Mine itself shows that the reported ‘dark’, ‘clear’ and porphyritic diorites formed within a span of less than one million years: the dark diorite (Ph-M2b) is dated at 3.42 My while the clear diorite (Ph-M5) and a porphyritic dacite (Ph-M6a) yield ages of 3.03 and 2.31 My, respectively. However the 3.42 My age is probably overestimated because the K2O content in this dark mineralized diorite is slightly too low compared with its neighbouring elements, as shown by its spidergram (figure 2). These are within the range of ages for Sto. Tomas (ST 34 Q1: 3.71 My) and the Clifton prospect (Clt 1: 2.33 My). Two thin andesitic dykes Ph-R6 and Ph-R1 sampled along the Philex Mine Road, cutting the Pugo Formation, dated at 2.45 My, also fall in the same ages range. These ages are compared in figure 3 with the previously reported ages, essentially for plutonic rocks, completed with unpublished ages of separated minerals (amphibole and feldspar) from these rocks. It shows that the feldspar ages derived from the plutonic rocks are younger by 1–2 My compared to the whole rock age, while the amphibole ages are older by more than 1–2 My. This suggests a cooling rate from 500 to 250 °C brought about by the intrusion of the later phases of magmatism and it can be concluded that the whole rock age, which is a mixed age, corresponds to the minimum age of emplacement.

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H. Bellon, G. P. Yumul Jr. / C. R. Acad. Sci. Paris, Sciences de la Terre et des planètes / Earth and Planetary Sciences 331 (2000) 295–302

*d

Baguio area

b

*a

Agno Antamok Acupan Virac Antamok Philex (Tailings Dam) Itogon

Clifton Philex Mine

Ansagan Halsema Road

Philex Mine Sto Tomas

Pugo MV Dalupirip (near Acupan)

0

5

10

New data Plut onic rock s Volc anic rock s

Me t a vo l c a n i c s

15

20

Previous data P l u t o ni c ro cks with Feld. WR

3.2. Geochemistry

Basic lavas (SiO2: 53–55 wt%) from the Pugo Metavolcanics and Dalupirip Schist display significant Nb and Zr negative anomalies typical of arc-related magmas, and a marked increase from Rb to Sm as do enriched MORB lavas (figure 2). We consider these coexisting features as the relevant signature of a backarc setting. Nevertheless, previous data for other lavas coming from these units lead to nearly flat spectra as for primitive island-arc magmatism [13]. Pliocene lavas and plutonic bodies from Philex Mine, for which silica contents range from 52 to 62 wt%, show within a time span of less than one million years a significant enrichment of K and other incompatible elements including light rare earth elements and a progressive depletion of heavy rare earth ones (figure 2). The porphyritic acidic andesite Ph-M6a, that is the last magmatic event recognized in the mine, is characterized by a low Yb content (1.1 ppm) and a Sr/Y ratio of 35, with a Y content of 9.8 ppm and so plots in the adakitic field in a Sr/Y versus Y diagram.

4. Discussion 4.1. Age and mode of formation of the Dalupirip Schist

The age and mode of formation of the Dalupirip Schist have always been an enigma. Nevertheless, the unconformable relationship between the Early Oligocene to early Early Miocene Zigzag Formation and the Pugo Metavolcanics suggests that the metavolcanics are at least Eocene in age. It follows that the protolith

300

25 Isotopic ages (Ma)

*

Amp. ages

New d at a

c

P r ev i o u s d at a

Black Mtn.

Figure 3. Previous and new isotopic ages (this work) for whole-rocks and separate minerals. Figure 3. Âges isotopiques publiés et nouveaux (ce travail) sur roche totale et minéraux séparés.

for the Dalupirip Schist is also Eocene in age, being the metamorphic equivalent of the Pugo Metavolcanics. The middle Early Miocene to early Middle Miocene ages (20–14.6 My) of the Dalupirip Schist that we derived, including the K–Ar age of 14.2 ± 2.2 My reported by Wolfe [19], are thermal rejuvenated ages. The next question that needs to be answered is what initiated this metamorphic event? It is difficult to imagine that shearing is purely responsible for the formation of this metamorphic unit considering its dimensions and distribution (figure 1). Regional metamorphism does not seem a viable mechanism because of the limited distribution of the Dalupirip Schist. Considering the disposition of this metamorphic rock unit with respect to igneous plutonic bodies and the reported presence of hornfels [10], it may be concluded that it formed through contact metamorphism. 4.2. Magmatic and tectonic controls on mineralization

As shown in figure 4, within a large magmatic period, as delimited by the clusters of different ages and remembering these whole-rock ages are minimum magmatic ages, the ‘at the end’ magmatic products are potassium enriched. At least, such enrichments are denoted ca. 14–13, 7 and 3 My. Whether this is a source region character or a process signature responsible for the magmatic pulses will have to be looked into in future works. Furthermore, the Miocene–Pliocene period is characterized by multiple phases of magma emplacement in the Baguio Mining District with contributions coming from both the proto-East Luzon Trough and the Manila Trench being duly recognized, e.g. [1]. The change in magmatic composition from tholeiitic through calc-

H. Bellon, G. P. Yumul Jr. / C. R. Acad. Sci. Paris, Sciences de la Terre et des planètes / Earth and Planetary Sciences 331 (2000) 295–302

time was marked by the subduction along the westdipping proto-East Luzon Trough–Philippine Trench which was followed by the subduction of the South China Sea crust along the east-dipping Manila Trench. The timing of this reversal is poorly constrained due mainly to the paucity of dated extrusive and intrusive rocks and the complex geological relationships of the exposed lithologies. The timing of the initiation of subduction of the South China Sea along the Manila Trench, as forwarded by previous workers, varies from Late Oligocene to Middle Miocene, e.g. [8, 14, 20].

3.0

K 2 O (wt%) 2.5

2.0

1.5

1.0

0.5

Isotopic ages (Ma) 0.0 0

1

2

3

4

5

6

7

LKCA Plutonic rocks Lavas

8

9

10 11

12

CA

13 14

15

HKCA

16 17 18

19 20

Sh

adakitic

Figure 4. Potassium enrichment through time of old and recent plutonism and recent volcanism. Magmatic series are LKCA: low potassium calc-alkaline lavas; CA: normal calc-alkaline lavas; HKCA: high potassium calc-alkaline lavas; and Sh: shoshonitic lavas. Figure 4. Enrichissement en potassium au cours du temps des plutons anciens et récents, ainsi que du volcanisme récent. Les séries magmatiques calco-alcalines, dont relèvent les échantillons, sont délimitées en fonction de leur richesse en potassium : faiblement potassique (LKCA), normale (CA), fortement potassique (HKCA) et shoshonitique (Sh).

alkaline to high-K calc-alkaline rocks [15] with adakitic rocks [13] is a characteristic of this long-lived subduction. The 4–2 My range in the Baguio Mining District is characterized by intense magmatism. Although this may be an artefact of sampling bias, this period is important since this is one of the events (another one being in Late Pliocene to Pleistocene) associated with gold-copper mineralization in the district. For that matter, previous workers concluded that the dominant control of mineralization in the area is tectonic. It is transected by laterformed shear zones related to extensional regimes which favor metal deposition. On the other hand, the multiplicity and intensity of magmatism is also believed to have helped in the mineralization of the district. Through the generation of ore-bearing magmatic pulses or the generation of a convection system due to the introduction of heat brought by magmatism, the chances for generating mineralization are increased. Add to this the fact that continuous intrusion of magmas results into the reactivation of faults and related structures which eventually could have become depositional sites of ore-bearing magmas. Mineralization, thus, was controlled by combined tectono-magmatic processes. 4.3. Arc polarity reversal in Northern Luzon

The geodynamic setting of Northern Luzon is characterized by arc polarity reversal. The Eocene–Oligocene

The uplift and erosion of the magmatic and sedimentary rocks that eventually formed part of the Klondyke Formation are attributed to the magma emplacement related to the subduction along the Manila Trench during a late Early Miocene period. A dacite boulder in the Klondyke Formation yielded an age of 20.2 ± 1.0 My [13] consistent with the early Middle to middle Late Miocene palaeontological age of the sedimentary matrix. Note that three other major events had transpired within that period: (i) the north–south opening of the east sub-basin of the South China Sea ceased ca. 17 My [18] and could have been followed by the initiation of subduction along the then newly formed Manila Trench, (ii) the proto-East Luzon Trough-related magmatism would be in its waning stage as indicated by potassic alkaline basaltic dikes in the Caraballo Range [13] dated at 17 My, and (iii) the Early Miocene reefal Kennon Limestone was being conformably deposited, with no magmatic product piercing through it and receiving no eroded magmatic materials from previous activities. These data suggest that arc polarity reversal and incipience of subduction along the Manila Trench occurred during Early Miocene (17–19 My) time. With this scenario, it is possible to find adakitic magmas of at least 16 to 15 My in age. This is brought about by the fact that slab melts form upon the subduction of young (<25 My), hot oceanic crusts or during the initiation of subduction. These two possibilities were present during the 19–17 My period for initiation of subduction along the Manila Trench. The apparent scarcity of adakitic volcanism may be attributed (i) to the possible emplacement of adakitic magmas low in the crust, or (ii) to the reaction of most of the adakitic magma within the mantle wedge or (iii) to the huge erosion of the effusive adakites. This has to be looked into in future work.

5. Conclusions Field, stratigraphic and isotopic datings suggest that arc polarity reversal and incipience of subduction along the Manila Trench of the South China Sea crust occurred during Early Miocene. The associated magmatism resulted into the uplift of the previous igneous and

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sedimentary series leading to their erosion and subsequent deposition as part of the Middle to Late Miocene Klondyke Formation. The Early Miocene period also witnessed the metamorphism of some portions of the Pugo Metavolcanics to form the Dalupirip Schist as a response to the intrusion of plutonic bodies and the linked formation of metamorphic aureoles: the 19.9 ± 1.2 to 14.7 ± 1.6 My ages derived for the Dalupirip Schist are interpreted as metamorphism reset ages.

Lastly, the 4 to 2 My period in the Baguio Mining District was characterized by a huge calc-alkaline magmatism with adakitic terms and gold-copper mineralization. Combined tectonic and magmatic controls aided in the accumulation and deposition of gold-copper deposits in the area. This scenario offers answers to several enigmatic questions that need to be known if we are to understand how this island arc system has evolved through time.

Acknowledgements. This is part of the ongoing France–Philippines cooperation program on the geosciences (Ophiolite Component). The French Ministry of foreign affairs is thanked for having supported the H B mission. Field support by D. Faustino, J. De Jesus, K. Kelly, P. Dalisay and laboratory support by J.C. Philippet (isotopic dating) and J. Cotten (ICP geochemical analyses) are acknowledged. The Philex Mine Corporation and Ms. R. Baluda are thanked for allowing access to their mine and sample collections. This paper was written during a DOST–ESEP post-doctoral fellowship by GPY at the University of Bretagne occidentale.

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