Abstract

Paleomagnetic studies in the Lachlan Orogen have indicated rotation of elements within it, but the evidence for rotation is limited and has significant statistical uncertainty. This study presents a detailed paleomagnetic investigation, assisted by X-ray analysis of magnetic mineralogy of middle Cambrian to lower Silurian sedimentary and igneous rocks in Victoria and New South Wales. Four demagnetisation behaviours (types A, B, C and D) have been recognised. Type A is characterised by a single component, resulting in an approximately linear path over the magnetite unblocking temperature range. Type B specimens exhibit slow progressive demagnetisation over a wide temperature range, with clustering of demagnetised directions above the magnetite Curie temperature, indicating hematite. Behaviour C is also characterised by clustering of directions, but this extends over a wider temperature range. Samples with behaviour D show noisy and non-informative demagnetisation, indicating very low magnetic stability. Hysteresis data confirm that samples with behaviour B and C have high coercivities and remain unsaturated above 300 mT. Mineral liberation analysis of selected samples indicates presence of iron oxides, magnetite, titanomagnetite, botryoidal hematite and iron hydroxides. Only three sites yielded specimens with type A primary magnetisation, with a poorly defined pole from the Dookie Volcanics on the central limb of the orocline, which does match a previous result from the same district. Two paleopoles, of type B and C specimens from the rocks in Tabberabbera Zone and the Rockley Volcanics, plot within the confidence limit of poles defining the late Silurian to Early Devonian Australian apparent polar wander path. These poles match other poles representing Devonian overprints around the margins of the Lachlan Orogen that have been interpreted to be the result of alteration mineralisation produced by fluids driven by tectonic compression during events in the Bindian (420–410 Ma) or early Tabberabberan (405–380 Ma) orogenies.

1. KEY POINTS

2. Paleomagnetic techniques have been used to test the rotation within the elements of the Lachlan Orocline in southeast Australia.

3. Results suggest that the original remanence in the rocks is obscured owing to post-deformation overprint.

4. This overprint is widespread in the Lachlan Orogen and is associated with the deformation during Bindian/Tabberabberan orogenic events.

摘要

拉克兰造山带的古地磁研究表明其中的元素发生了旋转，但旋转的证据有限并且具有显着的统计不确定性。本研究在维多利亚和新南威尔士州中寒武纪至志留纪下沉积岩和火成岩的 X 射线分析辅助下，进行了详细的古地磁调查。四种退磁行为（A、B、C 和 D 型）已得到认可。A 型的特点是单个组件，在磁铁矿畅通温度范围内产生近似线性的路径。B 型样品在很宽的温度范围内表现出缓慢的渐进退磁，在磁铁矿居里温度以上的退磁方向聚集，表明赤铁矿。行为 C 的特征还在于方向的聚类，但这延伸到更宽的温度范围。具有行为 D 的样品显示出噪声和无信息的退磁，表明磁稳定性非常低。滞后数据证实具有行为 B 和 C 的样品具有高矫顽力并且在 300 mT 以上保持不饱和。选定样品的矿物释放分析表明存在氧化铁、磁铁矿、钛磁铁矿、葡萄状赤铁矿和氢氧化铁。只有三个地点产生了具有 A 型初级磁化强度的样本，其极点来自于山脊中央边缘的 Dookie 火山，与之前同一地区的结果相符。来自 Tabberabbera Zone 和 Rockley Volcanics 岩石的 B 型和 C 型标本的两个古极，绘制在定义晚志留世至早泥盆世澳大利亚明显极地漂移路径的极点置信限内。这些极点与代表拉克兰造山带边缘周围泥盆纪叠印的其他极点相匹配，这些极点被解释为是在宾德（420-410 Ma）或塔伯拉贝兰早期（405- 380 Ma) 造山运动。

1. 关键点

2. 古地磁技术已被用于测试澳大利亚东南部 Lachlan Orocline 元素内的旋转。

3. 结果表明，由于变形后的叠印，岩石中的原始剩磁被掩盖了。

4. 这种叠印在拉克兰造山带普遍存在，并且与 Bindian/Tabberabberan 造山带事件期间的变形有关。