由40Ar / 39Ar测年揭示的蒸发岩分离的多卤石变形记录

IF 2.7 Q2 GEOCHEMISTRY & GEOPHYSICS
L. Richards, F. Jourdan, A. Collins, R. King
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引用次数: 2

摘要

摘要盐岭组是巴基斯坦北部的一个广泛的蒸发岩层序,它是容纳北部喜马拉雅造山变形的主要拆离。这种流变性弱的地层在Khewra矿中形成糜棱岩,可容纳约40公里位移,由插层的岩盐、钾盐和石膏泥灰岩组成。采用40Ar/39Ar步进加热激光和炉法,以钾泥灰岩和结晶岩盐样品中的多卤石[K2Ca2Mg(SO4)4⚫2H2O]颗粒作为地质时计,确定了矿物多卤石的形成年代和闭合温度。测定了两种多岩石样品的扩散特性:扩散系数(D0)、活化能(Ea)和%39Ar。这些值对应于闭合温度。在10°C的温度下,这两个样品的氩年龄测定结果都没有可靠的结晶年龄。这并不意外,因为多盐岩容易发生40ar *扩散损失,蒸发岩经历了许多阶段的变形,重新设置了封闭的K/Ar系统。样品06-3.1中最古老的最小加热步年龄为~ 514 Ma,相对较好地对应了该地层的早寒武纪年龄。样品05-P2和05-W2测量了阶跃年龄,并代表了一个变形事件,该事件部分重置了K/Ar系统,基于最古老的显著年龄在约381至415 Ma之间。我们解释了最年轻的测量步骤年龄,大约在286和292 Ma之间,代表变形诱导再结晶的最大年龄。样品05-P2和05-W2的最年轻和最古老的测量阶跃年龄都发生在该地区主要不整合的时间内。这些日期可能反映了在这一非沉积时期由大气水入渗和再结晶引起的K/Ar系统的部分重置。否则,它们可能是寒武纪降水后放射性衰变产生的Ar与普遍存在的新生代变形和物理再结晶产生的部分重置Ar混合的结果。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
Deformation recorded in polyhalite from evaporite detachments revealed by 40Ar ∕ 39Ar dating
Abstract. The Salt Range Formation is an extensive evaporite sequence in northern Pakistan that has acted as the primary detachment accommodating Himalayan orogenic deformation from the north. This rheologically weak formation forms a mylonite in the Khewra Mine, where it accommodates approximately 40 km displacement and is comprised of intercalated halite and potash salts and gypsiferous marls. Polyhalite [K2Ca2Mg(SO4)4⚫2H2O] grains taken from potash marl and crystalline halite samples are used as geochronometers to date the formation and identify the closure temperature of the mineral polyhalite using the 40Ar/39Ar step-heating laser and furnace methods. The diffusion characteristics measured for two samples of polyhalite are diffusivity (D0), activation energy (Ea), and %39Ar. These values correspond to a closure temperature of ca. 254 and 277 ∘C for a cooling rate of 10 ∘C Myr−1. 40Ar/39Ar age results for both samples did not return any reliable crystallisation age. This is not unexpected as polyhalite is prone to 40Ar* diffusion loss and the evaporites have experienced numerous phases of deformation resetting the closed K/Ar system. An oldest minimum heating step age of ∼514 Ma from sample 06-3.1 corresponds relatively well to the established early Cambrian age of the formation. Samples 05-P2 and 05-W2 have measured step ages and represent a deformation event that partially reset the K/Ar system based on oldest significant ages between ca. 381 and 415 Ma. We interpret the youngest measured step ages, between ca. 286 and 292 Ma, to represent the maximum age of deformation-induced recrystallisation. Both the youngest and oldest measured step ages for samples 05-P2 and 05-W2 occur within the time of a major unconformity in the area. These dates may reflect partial resetting of the K/Ar system from meteoric water infiltration and recrystallisation during this non-depositional time. Otherwise, they may result from mixing of Ar derived by radiogenic decay after Cambrian precipitation with partially reset Ar from pervasive Cenozoic deformation and physical recrystallisation.
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来源期刊
Geochronology
Geochronology Earth and Planetary Sciences-Paleontology
CiteScore
6.60
自引率
0.00%
发文量
35
审稿时长
19 weeks
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