Origin and emplacement of igneous rocks in the central Wasatch Mountains, Utah
Origin and emplacement of igneous rocks in the central Wasatch Mountains, Utah
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犹他州瓦萨奇山脉中部火成岩的起源和位置
DOI：
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发表时间：
2001
期刊：
影响因子：
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通讯作者：
K. Constenius
中科院分区：
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作者：
T. A. Vogel;F. W. Cambray;K. Constenius
瓦萨奇山脉中部的钙碱性火成岩的侵位时间为 36-30 Ma。它们形成了一个由十一个火山群和基特利火山场组成的带，沿着太古代怀俄明州和增生的古元古代地体之间的地壳缝合线排列。与该矿带相关的岩浆活动及其向西延伸至宾厄姆矿区的情况与新生代中期的扩张有关。根据质地，这些岩石由两种类型的岩石组成：西部类型，粗粒，大部分为等粒状；东部类型（包括基特利火山岩），细粒，斑状。西部种群（小三叶杨、阿尔塔和克莱顿峰种群）的成分变化形成了三个不同的成分组。东部种群的成分变化与阿尔塔种群的成分变化相似。这些岩石中的主要元素和微量元素变化类似于与俯冲相关的岩浆的变化。然而，高K2O含量和低ESR值与这个起源并不相符。这些岩浆是由镁铁质火成岩熔化形成的。我们认为，岩浆是由于白垩纪至新生代早期变形期间增厚的地壳重力塌陷而减压熔融而产生的。岩浆沿着东西走向的地壳上升到不同的高度。瓦萨奇山脉中部的火成岩的 eNd(t) 与大多数显生宙淡地斜线 (MG) 的火成岩相似，但 eSr(t) 明显较低。这种异常现象被解释为由于长期耗尽铷的地下室融化所致（Farmer 和 DePaolo，1983，1984）。然而，瓦萨奇火成岩带岩石是高钾钙碱性岩石，并且都具有非常相似的不相容微量元素模式，而只有少数MG岩石是钙碱性或高钾的。此外，在 MG 岩石中，不相容的微量元素模式是可变的。对于这些高钾、钙碱性岩石中铷长期耗尽的困境，一种可能的解释是，在塞维尔-拉拉米德事件（100-40 Ma）期间，地壳最近可能通过俯冲板片的脱水而充满了铷和钾。这一事件之后，造山带在新生代中期崩塌期间发生了融化（约 40-20 Ma）。岩浆的来源是下地壳中镁铁质岩石的熔化。其中一些岩浆积聚，形成岩浆房并发生分化。有些几乎没有积水，以基特利火山场的形式直接在地表喷发。持续的熔化和延伸从类似的地壳来源产生了新的岩浆。这些岩浆位于一系列与沿东西缝合线走滑位移相关的拉断结构下方。该缝合线可能受太古代-元古代边界控制。一些岩浆体很快就位到地表，没有发生明显的分异。其他人在很长一段时间内合并和分裂。这些岩浆体与地壳岩石相互作用，并分化为相对演化的成分。
The calc-alkaline igneous rocks in the central Wasatch Mountains were emplaced between 36–30 Ma. They form a belt comprised of eleven stocks and the Keetley volcanic field aligned along the crustal suture between the Archean Wyoming province and accreted Paleoproterozoic terranes. Magmatism associated with this belt and its westward continuation into the Bingham mining district has been related to mid-Cenozoic extension. These rocks consist of two types of stocks based on texture: a western type, which is coarse grained, and mostly equigranular, and an eastern type (including the Keetley volcanic rocks), which is fine grained and porphyritic. The compositional variation in the western stocks (Little Cottonwood, Alta, and Clayton Peak stocks) forms three distinct compositional groups. The compositional variation in the eastern stocks is similar to the compositional variation in the Alta stock. Major and trace element variations in these rocks resemble those of subduction-related magmas. However, the high K2O contents and low esr values are not consistent with this origin. These magmas formed from melting of mafic igneous rocks. We propose that magmas were generated by decompression melting due to gravitational collapse of the crust that had been thickened during Cretaceous to early Cenozoic deformation. Magmas rose to varying levels in the crust along an east–west lineament. The igneous rocks of the central Wasatch Mountains have eNd(t) similar to most of the Phanerozoic igneous rocks in the miogeocline (MG), but have significantly lower eSr(t). That anomaly has been explained as due to melting of a basement long depleted in Rb (Farmer and DePaolo, 1983, 1984). However, the Wasatch igneous belt rocks are high-potassium, calc-alkaline rocks and all have very similar incompatible trace element patterns, whereas only a few MG rocks are calc-alkaline or high potassium. Furthermore, in the MG rocks incompatible trace element patterns are variable. One possible explanation for the dilemma of long-time depletion of Rb in these high-potassium, calc-alkaline rocks is that the crust may have been recently charged with Rb and K during the Sevier-Laramide event (100–40 Ma) by dehydration of the subducting slab. This event was followed by melting during mid-Cenozoic collapse of the orogen (ca. 40–20 Ma). The source of the magmas was melting of mafic rocks in the lower crust. Some of these magmas ponded, formed magma chambers, and differentiated. Some involved little ponding and erupted directly on the surface in the form of the Keetley volcanic field. Continued melting and extension produced new magmas from a similar crustal source. These magmas were emplaced below a series of pull-apart structures associated with strike-slip displacement along an east–west suture. This suture may have been controlled by the Archean-Proterozoic boundary. Some magma bodies were emplaced quickly to the surface without significant fractionation. Others coalesced and fractionated over a protracted period of time. These magma bodies interacted with crustal rocks, and differentiated to relatively evolved compositions.