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Weiland Jr., Richard John, 1999 Emplacement of the Irian Ophiolite and unroofing of the Ruffaer Metamorphic Belt of Irian Jaya, Indonesia, PhD Dissertation, The University of Texas at Austin.
    © Richard John Weiland, Jr., 1999. Use of any part of this thesis for any purpose must be acknowledged.

Abstract

The distribution of Irian Ophiolite metabasites near the Gauttier Offset indicate exhumation from a northeast dipping subduction zone. Amphibolites, were metamorphosed at <5 kb and ∼700 °C, blueschists at ∼7 kb and ∼400 °C, and eclogites at ∼450 °C but not necessarily higher pressures. Basaltic compositions, REE concentrations, and radiogenic isotope ratios are characteristic of seawater altered ORB. Isotopic ages record metamorphism between 65 to 70 Ma and 50 to 45 Ma. Northern Ruffaer Metamorphic Belt metapelites have illite “crystallinity” values <0.25 °2&thetas;, paragonite and quartz, rare biotite and spessartine indicating peak temperatures of 300 to 350 °C. Whole-rock K-Ar ages range from 17 to 490 Ma, clustering between 35 and 20 Ma. Ages between 28 and 20 Ma correspond to samples containing lattice-oriented phyllosilicates and destruction of chlorite - white mica aggregates, recording metamorphism of passive margin strata. Intrusive rocks near the Irian Ophiolite are generally dioritic, medium- to low-K, subalkaline rocks. Trace element concentrations and radiogenic isotope ratios are characteristic of volcanic arc rocks. Isotopic ages range from ∼35 to 24 Ma and ∼12 to 10 Ma. The older suite is part of an allochthonous Oligocene - Early Miocene oceanic arc. The younger suite is coeval with the autochthonous Middle Miocene Maramuni Arc. Subduction of Australian passive margin strata and continental lithosphere led to uplift and of the Irian Ophiolite. Exhumation of metamorphic rocks primarily occurred by normal faulting near the ophiolite - metamorphic belt contact. Amphibolites were exhumed from <15 km depth, slate slivers from 15 to 20 km and phyllites from 25 to 30 km. Blueschists and eclogite were exhumed from 25 to 35 km depth along the Gauttier Offset. The forearc was unroofed at ≤ 0.5 km/m.y. Unroofing in the eastern metamorphic belt increased from ∼0.6 km/m.y. between 23 to 12 Ma, to ∼1.2 km/m.y. between 12 to 2 Ma and ∼3.8 km/m.y. since ∼2 Ma. The western metamorphic belt was unroofed at ∼0.3 km/m.y. from 21 to 3 Ma and ∼6.9 km/m.y.


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