Conference proceeding
Geology and petrology of the southeast Mariana forearc rift
American Geophysical Union Fall Meeting, Vol.2011
American Geophysical Union 2011 fall meeting
12/2011
Abstract
The southernmost Mariana convergent margin is tectonically and magmatically very active, with submarine arc volcanoes that are sub-parallel to the Malaguana-Gadao Ridge backarc spreading center at approximately 110km from the trench axis. This activity reflects widening of the S. Mariana Trough. Stretching formed 3 southeast-facing, broad rifts extending from the trench to an extinct arc volcano chain ( approximately 80km from the trench axis) that is mostly composed of outcrops and fragments of pillow lavas partially covered by sediments. The 3 rifts comprise the S.E. Mariana Forearc Rift (SEMFR) and are 50-56km long and 3600 to 8200m deep, with axial valleys that narrow near the extinct arc. We studied the SEMFR using one Shinkai 6500 dive in 2008 and two Shinkai 6500 dives and 7 deep-tows in 2010. Near the trench, the SEMFR flanks are very steep and dominated by talus slopes of lava, fine-grained gabbro, diabase and peridotite, sometimes covered by thin volcaniclastic sediments. Few outcrops of pillow lavas, lava flows and volcaniclastics are observed, strongly suggesting that SEMFR morphology is dominated by faulting and landsliding. Lava outcrops are smoother and better preserved towards the extinct arc, suggesting that magmatic activity dominates that part of the rift. (super 40) Ar- (super 39) Ar ages of 3 SEMFR lavas are 3.0-3.7 Ma, so post-magmatic rifting is younger than approximately 3Ma. SEMFR pillow lavas are vesicular and microporphyritic with crystallite-rich glassy rinds, indicating they erupted underwater at near-liquidus conditions. In contrast, the lava flows are more crystallized and less vesicular. SEMFR lavas exhibit similar ranges in mineral composition with 2 kinds of plagioclase (An>80% and An<80%), clinopyroxene (Mg#> or =80% and Mg#<80%), olivine (Fo>90 and Fo<90), suggesting magma mixing. Gabbroic rocks are slightly altered and have olivine and clinopyroxene compositions similar to those of the lavas, but contain less anorthitic plagioclase with a wider range in composition (An20-70) than the lavas. One sample of the extinct arc lava is vesicular and aphyric, consistent with near-liquidus eruption conditions. SEMFR lavas are low-K calk-alkaline basalts to andesites, whereas lava from the extinct arc is tholeiitic basaltic andesite similar to those erupted from the Malaguana-Gadao Ridge. These lavas are enriched in LILE and depleted in HFSE, consistent with subduction zone settings. Their similar Yb-normalized trace element patterns (Pearce et al., 2005), with different enrichment in LILE, suggest that SEMFR lavas were produced from a relatively homogenous mantle source that interacted with different proportions of subduction-derived components. Only SEMFR lavas sampled near the extinct arc exhibit different petrographic and geochemical features (higher abundances in olivine and clinopyroxene, lower HREE patterns, lower Nb/Yb and higher Ba/Th), indicating that a more depleted mantle source interacted with a greater subduction input.
Details
- Title: Subtitle
- Geology and petrology of the southeast Mariana forearc rift
- Creators
- Julia M Ribeiro - University of Texas at Dallas, Geosciences Richardson, TX USA United StatesElizabeth Y AnthonySherman H BloomerGuillaume GirardOsamu IshizukaKatherine A KelleyWilliam I MantonFernando MartinezSusan G MerleYasuhiko OharaMark K ReaganMinghua RenRobert J Stern
- Resource Type
- Conference proceeding
- Publication Details
- American Geophysical Union Fall Meeting, Vol.2011
- Conference
- American Geophysical Union 2011 fall meeting
- Publisher
- American Geophysical Union
- Alternative title
- AGU 2011 fall meeting
- Language
- English
- Date published
- 12/2011
- Academic Unit
- Earth and Environmental Sciences
- Record Identifier
- 9984240880602771
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