Protracted construction of gabbroic crust at a slow spreading ridge: Constraints from ²⁰⁶Pb/²³⁸U zircon ages from Atlantis Massif and IODP Hole U1309D (30°N, MAR)

Abstract

[1] Sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon ages of 24 samples from oceanic crust recovered in Integrated Ocean Drilling Program (IODP) Hole U1309D and from the surface of Atlantis Massif, Mid-Atlantic Ridge (MAR) (30°N) document a protracted history of accretion in the footwall to an oceanic detachment fault. Ages for 18 samples of evolved Fe-Ti oxide gabbro and felsic dikes collected 40-1415 m below seafloor in U1309D yield a weighted mean of 1.20 ± 0.03 Ma (mean square of weighted deviates = 7.1). However, the ages range from 1.08 ± 0.07 Ma and 1.28 ± 0.05 Ma indicating crustal construction occurred over a minimum of 100-200 ka. The zircon ages, along with petrologic observations, indicate at least 2 major periods of intrusive activity with age peaks separated by 70 ka. The oldest ages are observed below 600 mbsf, an observation inconsistent with models requiring constant depth melt intrusion beneath a detachment fault. The data are most consistent with a ''multiple sill'' model whereby sills intrude at random depths below the ridge axis over a length scale greater than 1.4 km. Zircon ages from broadly spaced samples collected along the southern ridge of Atlantis Massif yield a detachment fault slip rate of 28.7 ± 6.7 mm/a and imply significant asymmetric plate spreading (up to 100% on the North American plate) for at least 200 ka during core complex formation.

Tectonic and magmatic segmentation of the Global Ocean Ridge System: a synthesis of observations

“…These observations are consistent with tectonic thinning of the crust at the inside corner by long-lived (million years or more) detachment faults that can reach horizontal offsets of tens of kilometres and may account locally for 60 -100% of plate separation (e.g. Tucholke et al 1998;Ohara et al 2001;Searle et al 2003;Okino et al 2004;Buck 2005;Cannat et al 2006;Smith et al 2006;Baines et al 2008;Blackman et al 2008;Grimes et al 2008;Smith et al 2008). In the detachment fault model, lower crust and upper mantle rocks are exhumed to the seafloor at the inside corner and the volcanic section is spread to the outside corner (Dick et al 1981;Karson 1990;Tucholke & Lin 1994).…”

Late Cretaceous crustal growth in the Gangdese area, southern Tibet: Petrological and Sr–Nd–Hf–O isotopic evidence from Zhengga diorite–gabbro

“…Zircons in these samples have crystal lengths of 100-300 μm and length-to-width ratios from 1:1 to 2:1 (Fig. 3), which are similar to those from typical gabbros (Grimes et al, 2008). The zircon U-Pb isotopic data are given in Supplemental File 2.…”

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“…B. Example from Atlantis Massif: the southern wall is dominated by variably altered peridotites with gabbroic lenses (modified after Boschi et al, 2006) in contrast to (C) major gabbroic intrusions in the central dome (Grimes et al, 2008;copyright 2008 by the American Geophysical Union).…”