Publication Date:
2022-05-25
Description:
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.
Description:
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.
Description:
This study was supported by National Science Foundation Grant EAR-9910899.
Keywords:
Island arc crust
;
Layered gabbro
;
Alaska geology
;
Island arc magmatism
;
Lower crust
Repository Name:
Woods Hole Open Access Server
Type:
Preprint
Format:
application/pdf
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