Characteristics and petrogenesis of Alaskan-type ultramafic-mafic intrusions, Southeastern Alaska / Характеристика и петрогенезис ультрамафит-мафитовых интрузий аляскинского типа, юго-восточная Аляска

Alaskan-type ultramafic-mafic intrusions occur along a belt that extends from Duke Island to Klukwan in southeastern Alaska and fall into two age groups400 to 440 Ma; and 100 to 118 Ma. Most of the intrusions occur in the Alexander terrane or in the Gravina overlap assemblage, but they are not restricted to these terranes. The Alaskan-type ultramafic bodies range in size from sills only a few meters thick to intrusions about 10 km in maximum exposed dimension. Most of the bodies consist of magnetite-bearing hornblende clinopyroxenite or hornblendite, however many of the larger ones also include dunite, wehrlite, olivine clinopyroxenite, and, in some cases, gabbro. The Blashke Islands and Union Bay bodies are markedly concentrically zoned; dunite in the core is surrounded progressively outward by wehrlite, olivine clinopyroxenite, clinopyroxenite, hornblende clinopyroxenite, and gabbro. In the other large bodies, crude zoning may be present, but individual zones are discontinuous or missing entirely. 
Textural, mineralogical, and chemical characteristics of the Alaskan-type ultramafic bodies indicate that they formed from a basaltic magma by crystal fractionation and mineral concentration processes. In general the Mgl (Mg+Fe2+) ratio of olivine and clinopyroxene decreases systematically through the series dunite, wehrlite, olivine clinopyroxenite, clinopyroxenite, hornblende clinopyroxenite, and gabbro. The A1,0, content of clinopyroxene, which shows a marked enrichment with differentiation, suggests crystallization from progressively more hydrous melts like those characteristic of arc magmas. The hydrous nature of the magma is also indicated by the common occurrence of phlogopite and hornblende in wehrlite and clinopyroxenite and by hornblendite being part of the differentiation sequence. Clinopyroxene in the later differentiates has a substantial esseneite component that is a result of the hydrous, oxidizing nature of the magma. Rare-earth-element (REE) patterns of the ultramafic rock samples markedly show signatures of cumulus origin involving accumulation of dominantly olivine, clinopyroxene, and hornblende. The absence of a positive Eu anomaly and the relatively flat REE pattern of most gabbro samples suggest that the gabbros are not cumulates but probably represent static crystallization of a differentiated liquid that had undergone substantial removal of olivine,  linopyroxene, and some hornblende. The markedly similar REE abundance levels and patterns for the same rock types in all the bodies studied suggest that all the bodies were derived by differentiation of closely similar parent magmas under near-identical conditions  <...>