In-situ Raman spectroscopic measurements of lawsonite in the OH and H2O stretch-ing region were conducted up to approximately 2.5 GPa and 400 ℃ with a hydrothermal diamond-anvil cell. In the experimental design at room temperature, the ratio of band-area and the width at half maximum of the decomposed Raman band shows discontinuities at 0.8 GPa, and correspondingly, the 695-cm-1 peak in the low-wavenumber region was split into two peaks. In the experiments at high temperature, the splitting of the 695-cm-1 peak was induced at 2.4 GPa and 250 ℃. These findings demonstrate the existence of discontinuities in the pressure response of the strength of the hydrogen bonds, which suggests a possible change of the hydrogen-bonded structure in lawsonite under these conditions.
The Jiugequan ophiolite is one of the representative ophiolite fragments in the Early Paleozoic orogenic belt of the North Qilian Mountain.It has been drawn much attention and extensively studied in recent years.In this study,ion microprobe(SHRIMP) U-Pb dating was carried out for zircons from isotropic gabbro from the Jiugequan ophiolite.Eighteen analyses yield a relatively consistent apparent 206Pb/238U ages from 480 to 508 Ma with a weighted mean age of 490±5 Ma(MSWD=1.06),which is believed to be the crystallization age of the gabbro and thus the forming age of the Jiugequan ophiolite.Major and trace element geochemical study indicates that the diabase-basalts from the Jiugequan ophiolite have N-MORB and E-MORB characteristics with some subduction-related signatures.The petrological,geochemical and chronological data enable us to conclude that the Jiugequan ophiolite is most likely to be formed at a spreading center of back-arc basin during the early Ordovician,while the ancient Qilian oceanic plate subducted northwards.The acquisition of forming age and determination of tectonic setting for Jiugequan ophiolite provide significant constraints on the evolution of intra-oceanic subduction system in the North Qilian orogenic belt during the Early Paleozoic era.
