Oceanic and orogenic fluid-rock interaction in O-18/O-16-enriched metagabbros of an ophiolite (Tinos, Cyclades)

Putlitz, B Katzir, Y Matthews, A Valley, JW

B. Putlitz et al., Oceanic and orogenic fluid-rock interaction in O-18/O-16-enriched metagabbros of an ophiolite (Tinos, Cyclades), EARTH PLAN, 193(1-2), 2001, pp. 99-113

50

INGLESE

Article

Earth Sciences

EARTH AND PLANETARY SCIENCE LETTERS

0012-821X → ACNP

193

1-2

2001

99 - 113

ISI

0012-821X(20011130)193:1-2<99:OAOFII>2.0.ZU;2-G

Two spatial scales of fluid-rock interaction in an ophiolite suite are reve aled by oxygen isotope and hydrogen isotope studies of metagabbros on the i sland of Tinos (Cyclades, Greece). Sequentially formed mineral generations in the metagabbros include relict igneous augite, hornblende of sub-seafloo r hydrothermal origin, and actinolite and albite formed by regional greensc hist-facies metamorphism during orogenesis. With the exception of augite (d elta O-18 = 4.4-5.6 parts per thousand), the metagabbros are characterized by unusually high delta O-18 values: hornblende (5.8-7.4 parts per thousand ), actinolite (6.5-10.2 parts per thousand.), feldspar (14.6-14.9 parts per thousand )and whole rocks (7.0-10.5 parts per thousand). Hornblende deltaD values range from -57 to -66 parts per thousand. The high delta O-18 value s and the deltaD range of the hornblendes are compatible with interaction o f oceanic gabbro with seawater that had previously been enriched in O-18/O- 16 (delta O-18 = 6.5-8 parts per thousand) by isotopic exchange at moderate to high temperatures. The high degree of oceanic alteration in the layered gabbros, mass balance calculations of isotopic exchange, and field evidenc e for early oceanic thrusting suggest that seawater could have penetrated d eeply into the ocean crust, becoming O-18/O-16-enriched through isotopic ex change with gabbros at progressively increasing temperature. Upward, down-t emperature flow of the high-delta O-18 water would be very effective in ele vating the delta O-18 values of gabbros. The regional greenschist metamorph ic overprint of the ophiolite, possibly the result of continued thrusting a nd piling up of nappes during obduction, is characterized by localized flui d-rock exchange. Actinolite in massive gabbroic layers has delta O-18 value s (6.5-7.2 parts per thousand) close to those of the hornblende, whereas in deformed meter-sized gabbroic blocks the amphiboles have significantly hig her values (8.4-10.2 parts per thousand). Likewise, albite in the gabbroic blocks has high delta O-18 values of ca. 15 parts per thousand that are asc ribed to meter-scale exchange with O-18-rich fluids derived from dehydratio n reactions in low-temperature hydrothermally altered basaltic host rock en closing the blocks. Deformation-enhanced permeability facilitated fluid inf iltration in gabbroic blocks, whereas the relatively undeformed, and theref ore less permeable, massive gabbros experienced minor interaction with flui ds. The orogenic fluid-rock interaction thus represents local-scale redistr ibution of hydrous mineral components introduced during seafloor hydrotherm al exchange. (C) 2001 Elsevier Science BN. All rights reserved.