ABSTRACT We examine the pyrometamorphic changes and resulting crustal contamination that occurred when fragments of Otago Schist were entrained and pyrometamorphosed in Dunedin Volcanic Group (DVG) basanite. Otago Schist greenschist facies quartz-albite laminae were replaced by fine-grained quartz grains (inverted from β-quartz or tridymite) with orthopyroxene coronas and dacitic-rhyolitic glass. Former muscovite + chlorite + epidote ± garnet segregations were converted to plagioclase (An30-80), olivine, spinel, cordierite, ilmenite, pigeonite and trachytic to trachy-andesitic glass. Minerals largely formed by mineral replacement and then by crystal nucleation from a quenched liquid as protolith minerals passed their liquidi, with the latter process generating swallow-tailed, skeletal, dendritic and ladder-structured crystal morphologies. Orthopyroxene-, olivine- and plagioclase-liquid geothermometry indicates xenoliths reached at least 940°C. Despite the molten state of the xenoliths, discernible chemical and isotopic (Sr, Nd) contamination of the basanite was inhibited by its enriched trace element budget. The only evidence for crustal contamination comes from basanite Pb isotopes, which have less radiogenic compositions within several cm of the xenoliths. The subtle variation in DVG Sr and Nd isotopes may therefore be due to different mantle source components rather than crustal contamination by the underlying Otago Schist.

中文翻译：

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热变质奥塔哥片岩捕虏体对但尼丁火山群玄武岩造成轻微污染

摘要 我们研究了奥塔哥片岩碎片在但尼丁火山群 (DVG) 玄武岩中夹带和高温变质时发生的高温变质变化和由此产生的地壳污染。奥塔哥片岩绿片岩相石英-钠长石薄片被细粒石英颗粒（由β-石英或鳞石英转化而来）取代，带有斜方辉石冕和英安质-流纹质玻璃。以前的白云母 + 绿泥石 + 绿帘石 ± 石榴石偏析被转化为斜长石 (An30-80)、橄榄石、尖晶石、堇青石、钛铁矿、变辉石和粗晶到粗面安山岩玻璃。矿物主要是由矿物置换形成的，然后当原岩矿物通过它们的液体时，由淬火液体中的晶体成核形成，后一过程产生燕尾状、骨架状、树枝状和梯状结构的晶体形态。斜方辉石、橄榄石和斜长石液体地质测温表明捕虏体的温度至少达到 940°C。尽管捕虏体处于熔融状态，但其丰富的微量元素预算抑制了巴桑石可辨别的化学和同位素（Sr、Nd）污染。地壳污染的唯一证据来自玄武岩铅同位素，在捕虏体几厘米范围内具有较少的放射成分。因此，DVG Sr 和 Nd 同位素的细微变化可能是由于不同的地幔源成分，而不是下伏奥塔哥片岩的地壳污染。Nd) 巴桑石的污染受到其丰富的微量元素预算的抑制。地壳污染的唯一证据来自玄武岩铅同位素，在捕虏体几厘米范围内具有较少的放射成分。因此，DVG Sr 和 Nd 同位素的细微变化可能是由于不同的地幔源成分，而不是下伏奥塔哥片岩的地壳污染。Nd) 巴桑石的污染受到其丰富的微量元素预算的抑制。地壳污染的唯一证据来自玄武岩铅同位素，在捕虏体几厘米范围内具有较少的放射成分。因此，DVG Sr 和 Nd 同位素的细微变化可能是由于不同的地幔源成分，而不是下伏奥塔哥片岩的地壳污染。