Geochemistry of Tertiary Igneous Rocks of Northern Luzon, Philippines: Evidence for a Back-Arc Setting for Alkalic Porphyry Copper-Gold Deposits anda Case for Slab Roll-Back
Hollings, P and Wolfe, R and Cooke, DR and Waters, PJ, Geochemistry of Tertiary Igneous Rocks of Northern Luzon, Philippines: Evidence for a Back-Arc Setting for Alkalic Porphyry Copper-Gold Deposits anda Case for Slab Roll-Back, Economic Geology and The Bulletin of The Society of Economic Geologists, 106, (8) pp. 1257-1277. ISSN 0361-0128 (2011) [Refereed Article]
Oligocene to early Miocene volcanic rocks are preserved in the Central Cordillera range and Cagayan Valley
of northern Luzon, Philippines. Basaltic and andesitic rocks of the Pugo Formation in the Baguio district
of the Central Cordillera were intruded by the ~27 to 20 Ma calc-alkaline Central Cordillera Diorite Complex.
In the southern Cagayan Valley the subalkaline to alkaline late Oligocene Mamparang Formation overlies the
Cretaceous Caraballo Formation and has been intruded by the Didipio Igneous Complex, the Cordon Syenite
Complex, and the Palali batholith. The Didipio complex comprises an early suite of diorites, which were
intruded by the strongly mineralized stocks of the Dinkidi Cu-Au porphyry deposit. Whole-rock geochemical
data for intrusive and extrusive rocks of the Baguio district range from low K calc-alkaline to shoshonitic basalts
to dacites with rare earth element (REE) and high field strength element (HFSE) characteristics of suprasubduction
zone magmas and are all interpreted to have been sourced from the same parent melt. Samples from
Didipio display higher alkali contents but similar trace element characteristics. New age dates for the Didipio
area range from 25.7 to 24.8 Ma.
The potassic magmas of the Cagayan Valley are interpreted to have formed in a back-arc coeval to the mainarc
sequence that is preserved in the Baguio Miocene rocks. This contradicts earlier models, which invoke an
early Miocene arc reversal in the northern Luzon archipelago with the switch from early westward subduction
to later eastward subduction attributed to a variety of causes. The lack of a single compelling trigger for arc
reversal combined with the coeval emplacement of arc magmas in the west and back-arc magmas in the east
in northern Luzon is best interpreted as the result of eastward subduction since the late Oligocene. The presence
of ~20 Ma adakitic magmas in the Baguio district may indicate that flattening of the downgoing slab
resulted in a hiatus in magmatism and termination of back-arc rifting.