A detailed geochemical study of island arc crust : the Talkeetna Arc section, south–central Alaska
Greene, Andrew R.
Debari, Susan M.
Kelemen, Peter B.
Blusztajn, Jerzy S.
Clift, Peter D.
MetadataShow full item record
The Early to Middle Jurassic Talkeetna Arc section exposed in the Chugach Mountains of south central Alaska is 5-18 km wide and extends for over 150 km. This accreted island arc includes exposures of upper mantle to volcanic upper crust. The section comprises six lithologic units, in order of decreasing depth: (1) residual upper mantle harzburgite (with lesser proportions of dunite); (2) pyroxenite; (3) basal gabbronorite; (4) lower crustal gabbronorite; (5) mid-crustal plutonic rocks; and (6) volcanic rocks. The pyroxenites overlie residual mantle peridotite, with some interfingering of the two along the contact. The basal gabbronorite overlies pyroxenite, again with some interfingering of the two different units along their contact. Lower crustal gabbronorite (≤10 km thick) includes abundant rocks with well developed modal layering. The mid-crustal plutonic rocks include a heterogeneous assemblage of gabbroic rocks, dioritic to tonalitic rocks (30-40% area), and concentrations of mafic dikes and chilled mafic inclusions. The volcanic rocks (~7 km thick) range from basalt to rhyolite. Many of the evolved volcanic compositions are a result of fractional crystallisation processes whose cumulate products are directly observable in the lower crustal gabbronorites. For example, Ti and Eu enrichments in lower crustal gabbronorites are mirrored by Ti and Eu depletions in evolved volcanics. In addition, calculated parental liquids from ion microprobe analyses of clinopyroxene in lower crustal gabbronorites indicate that the clinopyroxenes crystallised in equilibrium with liquids whose compositions were the same as the compositions of volcanic rocks. The compositional variation of the main series of volcanic and chilled mafic rocks can be modeled through fractionation of observed phase compositions and phase proportions in lower crustal gabbronorite (i.e. cumulates). Primary, mantle-derived melts in the Talkeetna Arc underwent fractionation of pyroxenite at the base of the crust. Our calculations suggest that more than 25 wt % of the primary melts crystallised as pyroxenites at the base of the crust. The discrepancy between the observed proportion of pyroxenites (less than 5% of the arc section) and the proportion required by crystal fractionation modeling (more than 25%) may be best understood as the result of gravitational instability, with dense ultramafic cumulates, probably together with dense garnet granulites, foundering into the underlying mantle during the time when the Talkeetna Arc was magmatically active, or in the initial phases of slow cooling (and sub-solidus garnet growth) immediately after the cessation of arc activity.
Author Posting. © The Author, 2006. This is the author's version of the work. It is posted here by permission of Oxford University Press for personal use, not for redistribution. The definitive version was published in Journal of Petrology 47 (2006): 1051-1093, doi:10.1093/petrology/egl002.
Suggested CitationPreprint: Greene, Andrew R., Debari, Susan M., Kelemen, Peter B., Blusztajn, Jerzy S., Clift, Peter D., "A detailed geochemical study of island arc crust : the Talkeetna Arc section, south–central Alaska", 2005-12-23, https://doi.org/10.1093/petrology/egl002, https://hdl.handle.net/1912/1564
Showing items related by title, author, creator and subject.
Intermediate to felsic middle crust in the accreted Talkeetna arc, the Alaska Peninsula and Kodiak Island, Alaska : an analogue for low-velocity middle crust in modern arcs Rioux, Matthew; Mattinson, James; Hacker, Bradley R.; Kelemen, Peter B.; Blusztajn, Jerzy S.; Hanghoj, Karen; Gehrels, George (American Geophysical Union, 2010-05-08)Seismic profiles of several modern arcs have identified thick, low-velocity midcrustal layers (Vp = 6.0–6.5 km/s) that are interpreted to represent intermediate to felsic plutonic crust. The presence of this silicic crust ...
The stability of arc lower crust : insights from the Talkeetna Arc section, south-central Alaska and the seismic structure of modern arcs Behn, Mark D.; Kelemen, Peter B. (American Geophysical Union, 2006-11-11)One process for the formation of continental crust is the accretion of arc terranes at continental margins. A longstanding problem with this model is that although the composition of the continental crust is andesitic, ...
Hacker, Bradley R.; Mehl, Luc; Kelemen, Peter B.; Rioux, Matthew; Behn, Mark D.; Luffi, Peter (American Geophysical Union, 2008-03-07)The Talkeetna arc is one of two intraoceanic arcs where much of the section from the upper mantle through the volcanic carapace is well exposed. We reconstruct the vertical section of the Talkeetna arc by determining the ...