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Itogon Quartz Diorite  

Lithology

Hornblende quartz diorite, tonalite, minor gabbro

Stratigraphic relations

Intrudes Pugo, Zigzag and Lepanto formations

Distribution

Mankayan, Benguet; Bontoc area; Baguio District

Age

Middle Miocene

Named by

Schafer (1954)

Synonymy

Kelly Diorite (Schafer, 1954); Bagon Intrusive (Sillitoe and Angeles, 1985)

The Itogon Quartz Diorite was one of the diorite bodies named by Schafer (1954) for the plutonic bodies around the Baguio District.  The other intrusives named by Schafer (1954) are: Antamok Diorite, ViracGranodiorite and Kelly Diorite, of which Kelly seems equivalent to the Itogon Quartz Diorite.  These also comprise the Agno Batholith of Fernandez and Pulanco (1967).  The bulk of the quartz diorite bodies consist of hornblende quartz diorite although diorites with smaller amounts of quartz or none at all are known to occur.  At Philex Mine at Padcal, Tuba, Benguet, the quartz diorite is fringed by gabbro.  The numerous quartz diorite clasts found in the Klondyke Formation are derived from this intrusive body.

As indicated by radiometric dating, there are two main pulses of plutonic intrusions in the region, the earlier phase being the Oligocene Central Cordillera Diorite Complex and the later phase represented by the Middle Miocene (12-15 Ma) Itogon Quartz Diorite (Wolfe, 1981; Maleterre, 1989).  Fission track dating of zircons from a sample of quartz diorite near Philex Mine gave 15 Ma, which agrees with the K/Ar dating of 14.8 Ma for the same sample (Lovering, 1983).  Sillitoe and Angeles (1985) cite a K/Ar dating of biotite (12 ± 0.4 Ma) and hornblende (13 ± 0.8 Ma) from tonalite intruding conglomerates at Mankayan, Benguet, confirming a Middle Miocene emplacement of some of the intrusives.  This includes the Bagon Intrusive in Lepanto area.

Formation sequenceEdit

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