Deep crustal felsic xenoliths from classic Scottish Midland Valley localities, carried to the surface by Permo-Carboniferous magmatism, are shown for the first time to include metaigneous varieties with dioritic and tonalitic protoliths. Four hypotheses regarding their origin have been evaluated: (1) Precambrian basement; (2) Permo-Carboniferous underplating; (3) ‘Newer Granite’ magmatism; (4) Ordovician arc magmatism. U–Pb zircon dating results rule out the Precambrian basement and Permo-Carboniferous underplating hypotheses, but establish that the meta-igneous xenoliths represent both ‘Newer Granite’ and Ordovician (to possibly Silurian) arc magmatism. The metadiorite xenoliths are shown to have protolith ages of c. 415 Ma with εHf_t zircon values ranging from +0·1 to +11·1. These are interpreted to represent unexposed ‘Newer Granite’ plutons, based on age, mineralogical, isotopic and geochemical data. This shows that Devonian ‘Newer Granite’ magmatism had a greater impact on the Midland Valley and Southern Uplands crust than previously realized. Clinopyroxene–plagioclase–quartz barometry on the metadiorites from the east and west of the Midland Valley yielded a similar pressure range of c. 5–10 kbar, and a metadiorite from the east yielded a minimum two-feldspar temperature estimate of c. 793–816°C. These results indicate that the metadiorites once resided in the middle–lower crust. In contrast, two metatonalite xenoliths have a Late Ordovician protolith age (c. 453 Ma), with zircon εHf_t values of +7·8 to +9·0. These are interpreted as samples of a buried Late Ordovician magmatic arc situated within the Midland Valley. Inherited zircons with similar Late Ordovician ages and εHf_t=453 values (+1·6 to +10·8) are present in the metadiorites, suggesting that the Devonian ‘Newer Granites’ intruded within or through this Late Ordovician Midland Valley arc. A younger protolith age of c. 430 Ma from one of the metatonalites suggests that arc activity continued until Silurian times. This validates the long-standing ‘arc collision’ hypothesis for the development of the Caledonian Orogen. Based on U–Pb zircon dating, the metatonalite and metadiorite xenoliths have both experienced metamorphism between c. 400 and c. 391 Ma, probably linked to the Acadian Orogeny. An older phase of metamorphism at c. 411 Ma was possibly triggered by the combined effects of heating owing to the emplacement of the ‘Newer Granite’ plutons and the overthrusting of the Southern Uplands terrane onto the southern margin of the Midland Valley terrane.

中文翻译：

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苏格兰米德兰河谷深地壳火成岩异岩的年龄和起源：早期古生代弧和“新花岗岩”岩浆作用的遗迹

来自Permit-Carboniferous岩浆作用带到地表的来自苏格兰经典米德兰河谷地区的深地壳长英质异质岩首次显示出含有闪长生质和成色质原生质的亚火成岩变种。关于其起源的四个假设已得到评估：（1）前寒武纪基底；（2）碳-石炭纪的底层；（3）“新花岗岩”岩浆作用；（4）奥陶纪弧岩浆作用。U–Pb锆石测年结果排除了前寒武纪基底和二叠纪-石炭纪的地下沉积假说，但确定了亚火成的异岩岩既代表了“新花岗岩”又代表了奥陶纪（甚至可能是志留纪）弧岩浆作用。闪长岩异种岩被证明具有c的原生石年龄。415马与εHf _Ť锆石值范围为+ 0·1至+ 11·1。根据年龄，矿物学，同位素和地球化学数据，这些被解释为代表未暴露的“新花岗岩”岩体。这表明泥盆纪的“新花岗岩”岩浆作用对米德兰河谷和南部高地地壳的影响比以前意识到的要大。来自米德兰谷东部和西部的闪长岩上的斜辉石-斜长石-石英气压计产生了相似的压力范围c。5-10 kbar，从东部的闪闪发光产生的最低两长石温度估计为c。793–816°C。这些结果表明，该闪长岩曾经居住在中下地壳中。相比之下，二metatonalite捕虏体有一个晚奥陶世原岩年龄（约453马），锆石εHf _Ť值从+ 7·8到+ 9·0。这些被解释为位于米德兰谷地内的一个晚期奥陶纪岩浆弧的样本。继承锆石与同类晚奥陶世年龄和εHf_{牛逼= 453}闪长岩中存在的值（+ 1·6至+ 10·8），表明泥盆纪“较新的花岗岩”侵入了奥陶纪中部谷地弧内或穿过该弧。较年轻的原生石时代c。来自其中一种变质岩的430 Ma表明弧活动一直持续到志留纪时期。这验证了加里东造山带发展的长期“弧碰撞”假说。根据U–Pb锆石年代，变质岩和变闪长岩异岩都经历了c之间的变质作用。400和c。391 Ma，可能与Acadian造山运动有关。在c的变质的较早阶段。411 Ma可能是由于“较新的花岗岩”岩体的到位和南部高地地块向米德兰河谷地块南缘的强推作用共同造成的。