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Central Cordillera Diorite Complex

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Central Cordillera Diorite Complex

Lithology

Hornblende quartz diorite, tonalite, granodiorite, quartz monzodiorite, pyroxene-bearing diorite, hornblende diorite, monzodiorite, minor gabbro

Stratigraphic relations

Intrudes Lepanto, Pugo, and Malitep Formations

Distribution

Mankayan, Benguet; Bontoc area; Baguio District

Age

Late Oligocene

Previous Name

Agno Batholith (Fernandez and Pulanco, 1967)

Renamed by

Yumul and others (1992)

The batholithic intrusions of intermediate composition (diorite, quartz diorite, granodiorite) in incised valleys and mountains constituting the spine of Central Cordillera, was designated as Agno Batholith by Fernandez and Pulanco (1967).  Previously, these intrusions were named after specific localities in Baguio District by Schafer (1954) such as Antamok Diorite, Virac Granodiorite, Kelly Diorite and Itogon Quartz Diorite.  Because of the dissimilarities in periods of intrusion, Wolfe (1981) proposes to name the Ologocene intrusive bodies (mostly in northern Cordillera) as Cordillera Batholith and the younger diorites (mostly occupying the west flank of the Cordillera in the south) as Agno Pluton.  However, because of the lack of criteria for distinguishing one form the other in the field (except where they intrude Miocene rocks), these dioriticintrusives were lumped together by Yumul and others (1992) into a single unit, which they named Central Cordillera Diorite Complex.  It consists mainly of intermediate rocks such as hornblende quartz diorite, tonalite, granodiorite, quartz monzodiorite, pyroxene-bearing diorite, hornblende diorite, monzodiorite, with minor alkaline gabbro and quartz gabbro.  The bulk of the diorite complex consists of hornblende quartz diorite.  They are mostly intrusive into the Pugo Formation .  Few clasts of dioritic rocks were noted in Zigzag Formation .

Here, the Central Cordillera Diorite Complex is considered an earlier pulse of plutonic intrusion in the region as distinguished from a later phase represented by the Itogon Quartz Diorite (see below).  Wolfe (1981) reports an average dating of 27 Ma (Late Oligocene) representing the earlier phase and 12-15 Ma for the later phase.  Maleterre (1989) reports values of 29 Ma and 30.6 Ma for samples near Bontoc that correspond to Late Oligoceneplutonism postulated by Wolfe (1981).  Encarnacion and others (1993) report a zircon U-Pb dating of 26.8 ± 0.4 Ma for a quartz diorite sample taken east of Baguio City, about 2 km west of the Agno River.Plutonism could have extended to Early Miocene as indicated by K-Ar dating of 16-20 Ma (Maleterre, 1989).

Formation sequenceEdit

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