Granodiorite and related rocks

Unit symbol: Ogi
Age range Ordovician (477.7 to 443.4 Ma)
Lithology: Igneous - Plutonic
Group name: Older plutonic rocks of southeast Alaska
Part of the plutonic complex of southern Prince of Wales Island; unit consists of foliated and locally layered leucocratic granodiorite that is associated with diorite and subordinate hypabyssal diorite, gabbro, and rocks described as leucogranodiorite by Gehrels (1992) and as alaskite by Herreid and others (1978). Diorite is medium grained and consists of green hornblende locally intergrown with brown biotite, altered plagioclase, and subordinate quartz and K-feldspar; it is moderately layered or foliated and locally cataclastic (Gehrels, 1992). Granodiorite is fine-grained, and has a foliation that is defined by elongate hornblende and quartz grains. Highly foliated rocks are generally confined to narrow domains that may have been ductile shear zones. Contact relations and variability of foliation indicate granodiorite was emplaced and deformed prior to complete crystallization of diorite (Gehrels, 1992). Leucogranodiorite contains interlocking and commonly myrmekitic plagioclase, quartz, and microperthite; plagioclase is commonly altered to white mica, calcite, and epidote. Ferromagnesian minerals are totally altered to chlorite and opaque grains. Intrudes the diorite and rocks interpreted to be the Descon Formation (part of map unit SOd); overlain by Karheen Formation (Gehrels, 1992). These plutons yield U/Pb ages in the range of 472 to 445 Ma (Gehrels, 1992)

Source map information

Source map Gehrels, G.E., 1992, Geologic map of southern Prince of Wales Island, southeastern Alaska: U.S. Geological Survey Miscellaneous Investigations Series Map I-2169, 23 p., 1 sheet, scale 1:63,360.
Symbol Oqd
Unit name Quartz diorite and diorite
Description Fine- to medium-grained quartz diorite, and subordinate diorite and quartz monzonite. Consists of interlocking plagioclase, hornblende, and quartz. Tabular plagioclase grains range from 1 to 6 mm. Grains are slightly zoned with interiors altered to white mica, calcite, and epidote. Less than 15 per cent quartz. Intrudes Descon Formation, and intruded by SOqm.
Lithology Igneous

Correlated geologic units

Label Ogd
Description Granodiorite on Prince of Wales Island
Geologic age Floian to Rhuddian
Geologic setting Intrusive
Lithology Form Importance
Quartz-diorite < Dioritic < Plutonic < Igneous Pluton Major
Granodiorite < Granitic < Plutonic < Igneous Pluton Major
Alaskite < Leucocratic-granitic < Granitic < Plutonic < Igneous Pluton Minor
Quartz-monzonite < Syenitic < Plutonic < Igneous Pluton Minor
Gabbro < Gabbroic < Plutonic < Igneous Pluton Incidental
Orthogneiss < Metaigneous < Metamorphic Hornfels Incidental