Argon, uranium, thorium and lead composition and ages of ODP Leg 180 sites@en

During Ocean Drilling Program (ODP) Leg 180, 11 sites were drilled in the vicinity of the Moresby Seamount to study processes associated with the transition from continental rifting to seafloor spreading in the Woodlark Basin. This paper presents thermochronologic (40Ar/39Ar, 238U/206Pb, and fission track) results from igneous rocks recovered during ODP Leg 180 that help constrain the latest Cretaceous to present-day tectonic development of the Woodlark Basin. Igneous rocks recovered (primarily from Sites 1109, 1114, 1117, and 1118) consist of predominantly diabase and metadiabase, with minor basalt and gabbro. Zircon ion microprobe analyses gave a 238U/206Pb age of 66.4 ± 1.5 Ma, interpreted to date crystallization of the diabase. 40Ar/39Ar plagioclase apparent ages vary considerably according to the degree to which the diabase was altered subsequent to crystallization. The least altered sample (from Site 1109) yielded a plagioclase isochron age of 58.9 ± 5.8 Ma, interpreted to represent cooling following intrusion. The most altered sample (from Site 1117) yielded an isochron age of 31.0 ± 0.9 Ma, interpreted to represent a maximum age for the timing of subsequent hydrothermal alteration. The diabase has not been thermally affected by Miocene-Pliocene rift-related events, supporting our inference that these rocks have remained at shallow and cool levels in the crust (i.e., upper plate) since they were partially reset as a result of middle Oligocene hydrothermal alteration. These results suggest that crustal extension in the vicinity of the Moresby Seamount, immediately west of the active seafloor spreading tip, is being accommodated by normal faulting within latest Cretaceous to early Paleocene oceanic crust. Felsic clasts provide additional evidence for middle Miocene and Pliocene magmatic events in the region. Two rhyolitic clasts (from Sites 1110 and 1111) gave zircon 238U/206Pb ages of 15.7 ± 0.4 Ma and provide evidence for Miocene volcanism in the region. 40Ar/39Ar total fusion ages on single grains of K-feldspar from these clasts yielded younger apparent ages of 12.5 ± 0.2 and 14.4 ± 0.6 Ma due to variable sericitization of K-feldspar phenocrysts. 238U/206Pb zircon, 40Ar/39Ar K-feldspar and biotite total fusion, and apatite fission track analysis of a microgranite clast (from Site 1108) provide evidence for the existence of a rapidly cooled 3.0 to 1.8 Ma granitic protolith. […]大洋钻探计划（Ocean Drilling Program, ODP）第180航次期间，研究人员在莫尔斯比海山（Moresby Seamount）周边钻取了11个站位，旨在探究伍德拉克盆地（Woodlark Basin）从大陆裂谷作用转向海底扩张的相关过程。本研究报道了ODP第180航次采获的火成岩的热年代学（thermochronologic）测试结果，包括40Ar/39Ar、238U/206Pb以及裂变径迹（fission track）分析，这些数据可为约束伍德拉克盆地白垩纪末期至今的构造演化提供依据。采获的火成岩主要取自1109、1114、1117和1118站位，以辉绿岩（diabase）和变辉绿岩（metadiabase）为主，伴生少量玄武岩（basalt）与辉长岩（gabbro）。对锆石进行离子探针分析后得到238U/206Pb年龄为66.4±1.5 Ma，该年龄被解释为辉绿岩的结晶年龄。斜长石的40Ar/39Ar表观年龄随辉绿岩结晶后蚀变程度的不同而存在显著差异：蚀变程度最低的样品（采自1109站位）给出的斜长石等时线年龄为58.9±5.8 Ma，该年龄被认为代表岩体侵入后的冷却年龄；而蚀变程度最高的样品（采自1117站位）的等时线年龄为31.0±0.9 Ma，被认为代表后续热液蚀变发生的最大年龄。辉绿岩并未受到中新世-上新世裂谷作用相关热事件的影响，这一结果支持了我们的推论：自渐新世中期热液蚀变导致岩体部分热重置以来，这些岩石一直处于地壳浅部低温环境（即上盘板块）中。上述结果表明，位于活跃海底扩张尖端西侧紧邻区域的莫尔斯比海山周边地壳伸展作用，是通过白垩纪末期至古近纪早期洋壳内的正断层活动得以调节的。长英质岩屑为该区域中新世中期和上新世的岩浆活动提供了额外证据：两个采自1110和1111站位的流纹质岩屑，其锆石238U/206Pb年龄为15.7±0.4 Ma，证实了该区域存在中新世火山活动；对这些岩屑中单颗粒钾长石（K-feldspar）进行40Ar/39Ar全熔年龄测试，由于钾长石斑晶存在不同程度的绢云母化作用，得到的表观年龄更低，分别为12.5±0.2 Ma和14.4±0.6 Ma。对采自1108站位的微花岗岩岩屑开展的锆石238U/206Pb、钾长石与黑云母（biotite）40Ar/39Ar全熔分析以及磷灰石裂变径迹测试结果表明，该区域存在一个冷却速率极快的3.0~1.8 Ma花岗质原岩。[…]