title: Jurassic zircons from the Southwest Indian Ridge.
abstract: The existence of ancient rocks in present mid-ocean ridges have long been observed but received less attention. Here we report the discovery of zircons with both reasonably young ages of about 5 Ma and abnormally old ages of approximate 180 Ma from two evolved gabbroic rocks that were dredged from the Southwest Indian Ridge (SWIR) in the Gallieni fracture zone. U-Pb and Lu-Hf isotope analyses of zircons were made using ion probe and conventional laser abrasion directly in petrographic thin sections. Young zircons and their host oxide gabbro have positive Hf isotope compositions (εHf = +15.7-+12.4), suggesting a highly depleted mantle beneath the SWIR. The spread εHf values (from-2.3 to-4.5) of abnormally old zircons, together with the unradiogenic Nd-Hf isotope of the host quartz diorite, appears to suggest an ancient juvenile magmatism along the rifting margin of the southern Gondwana prior to the opening of the Indian Ocean. A convincing explanation for the origin of the unusually old zircons is yet to surface, however, an update of the theory of plate tectonics would be expected with continuing discovery of ancient rocks in the mid-oceanic ridges and abyssal ocean basins.
authors.affiliation: State Key Laboratory of Marine Geology, Tongji University, Shanghai 200092, China.
authors.forename: Hao
authors.initials: H
authors.lastname: Cheng
authors.affiliation: State Key Laboratory of Marine Geology, Tongji University, Shanghai 200092, China.
authors.forename: Huaiyang
authors.initials: H
authors.lastname: Zhou
authors.affiliation: State Key Laboratory of Marine Geology, Tongji University, Shanghai 200092, China.
authors.forename: Qunhui
authors.initials: Q
authors.lastname: Yang
authors.affiliation: State Key Laboratory of Marine Geology, Tongji University, Shanghai 200092, China.
authors.forename: Lingmin
authors.initials: L
authors.lastname: Zhang
authors.affiliation: State Key Laboratory of Marine Geology, Tongji University, Shanghai 200092, China.
authors.forename: Fuwu
authors.initials: F
authors.lastname: Ji
authors.affiliation: State Key Laboratory of Marine Geology, Tongji University, Shanghai 200092, China.|Woods Hole Oceanographic Institution, Woods Hole, MA 02543-1050, USA.
authors.forename: Henry
authors.initials: H
authors.lastname: Dick