PETROGENETIC EVALUATION OF THE DUKE ISLAND COMPLEX, AN ALASKAN-TYPE ULTRAMAFIC INTRUSION IN SOUTHEASTERN ALASKA

Alaskan-type complexes are ultramafic, typically alkaline to sub-alkaline in composition and are arranged in linear zones along the trend of major orogenic belts or convergent plate margins. These intrusions have small aerial extents and show concentric zoning from dunite or peridotite cores to less mafic rocks outwards. The Duke Island Complex in southeastern Alaska is one such intrusion. The lithology of the intrusion varies from dunite and wehrlite at the core followed by zones of olivine clinopyroxenite and hornblende-magnetite clinopyroxenite towards the periphery. As in most Alaskan-type intrusions, the Duke Island Complex is surrounded by a thick envelope of gabbro. But unlike other Alaskan-type complexes Duke Island hosts zones of sulfide segregations.

Olivine pyroxenite is the predominant rock type that grades into hornblende magnetite pyroxenite towards the margin of the complex. The pyroxene is diopsidic augite in composition ranging from Di₇₀Hd₃₀ in the pyroxenites to Di₉₀Hd₁₀ in the peridotites and dunites. The composition of olivine varies from Fo₇₅ in olivine pyroxenites to Fo₉₅ in the dunites. Plagioclase is absent in the entire ultramafic part of the complex. It is present only in the pegmatitic hornblendites along the contact zone of the complex with the surrounding gabbroic rocks.

δ¹⁸O values in the clinopyroxenes range between 5 and 6 ‰ and do not indicate significant crustal assimilation but a large scatter in δ³⁴S values between +5 and -14 ‰ clearly indicate the involvement of external sulfur. The REE patterns distinguish different lithological units of the complex based on the relative abundances of olivine, clinopyroxene and hornblende and attest to the formation of the rock types by crystal accumulation of these minerals. Similarity in trace element patterns between different Alaskan-type intrusions in south-eastern Alaska also suggests that assimilation of country rock must have been minimal.

The lithological zoning in the complex can be explained by fractional crystallization and subsequent diapiric updoming of unconsolidated crystal layers with the expulsion of the fractionated liquid. Conspicuous grain-size layering in olivine clinopyroxenite indicates convection and crystal settling in the magma chamber accompanied by accumulation of the sulfide liquid.