Crystal accumulation in a tilted arc batholith
Crystal accumulation in a tilted arc batholith
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DOI：
10.2138/am-2016-5404
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发表时间：
2016-08
影响因子：
3.1
通讯作者：
Calvin G. Barnes;N. Coint;A. Yoshinobu
中科院分区：
文献类型：
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作者：
Calvin G. Barnes;N. Coint;A. Yoshinobu
摘要 伍利溪基岩是位于加利福尼亚州克拉马斯山省的晚侏罗世、与弧相关的钙碱性深成杂岩体。就位后的倾斜和侵蚀暴露了约 12 公里的结构起伏。该杂岩体由许多岩浆分批侵位而成的较古老（~159.1 Ma）下部区域（辉石岩到英辉长岩）、较年轻（~158.2 Ma）上部区域（石英闪长岩到花岗岩）和过渡中心区域组成。在下部区域，辉石的铁含量太高，无法与母岩成分的熔体达到平衡。使用流纹岩熔体进行的质量平衡计算和模拟表明，这些岩石是辉石和斜长石、橄榄石以及副磷灰石和氧化物的堆积物。间隙熔化的百分比变化范围为~7.5-83%。在大多数镁铁质岩石中，堆积物的斜长石/辉石比率变化很大，但在石英闪长岩到英闪长岩中相对均匀。这种接近恒定的比率导致模拟液体下降线的成分趋势。在上部区域，块体岩石的成分趋势与安山母岩浆的分异一致。通过质量平衡计算和流纹岩-熔体模拟模拟，从石英闪长岩到花岗岩成分的向上级配表明，上部区域的结构下部是角闪石和斜长石±黑云母和副矿物的堆积物，其中 37-80% 被困熔体。相比之下，上部区域结构较高的部分代表了逃离下方堆积体的差异化岩浆，代表了剩余 92-54% 的差异化熔体分数。在上部区域堆积物中，堆积斜长石/（斜长石+镁铁矿物）的比率约为0.48，模拟液体下降线。结果表明，许多钙碱性岩体的成分变化可能至少与分异结晶一样代表晶体积累。如果是这样，那么弧岩体在地球化学上类似于冷冻液体的假设是值得怀疑的，应该根据具体情况进行检验。此外，除非考虑到深成岩中的堆积，否则将深成岩成分与潜在的共岩浆火山岩成分进行比较通常会产生虚假结果。
Abstract The Wooley Creek batholith is a Late Jurassic, arc-related, calc-alkaline plutonic complex in the Klamath Mountain province of California. Post-emplacement tilting and erosion have exposed ~12 km of structural relief. The complex consists of an older (~159.1 Ma) lower zone (pyroxenite to tonalite) assembled by piecemeal emplacement of many magma batches, a younger (~158.2 Ma) upper zone (quartz diorite to granite), and a transitional central zone. In the lower zone, pyroxenes are too Fe rich to be in equilibrium with a melt whose composition was that of the host rock. Mass-balance calculations and simulations using rhyolite-MELTS indicate that these rocks are cumulates of pyroxenes and plagioclase ± olivine and accessory apatite and oxides. Percentages of interstitial melt varied from ~7.5–83%. The plagioclase/pyroxene ratios of cumulates vary considerably among the most mafic rocks, but are relatively uniform among quartz diorite to tonalite. This near-constant ratio results in compositional trends that mimic a liquid line of descent. In the upper zone, bulk-rock compositional trends are consistent with differentiation of andesitic parental magmas. Upward gradation from quartz dioritic to granitic compositions, modeled via mass-balance calculations and rhyolite-MELTS simulations, indicate that structurally lower parts of the upper zone are cumulates of hornblende and plagioclase ± biotite and accessory minerals, with 37–80% trapped melt. In contrast, the structurally higher part of the upper zone represents differentiated magma that escaped the subjacent cumulates, representing differentiated melt fractions remaining from 92–54%. The ratio of cumulate plagioclase/(plagioclase + mafic minerals) is ~0.48 among upper-zone cumulates, mimicking a liquid line of descent. The results suggest that compositional variation in many calc-alkaline plutons may be at least as representative of crystal accumulation as of fractional crystallization. If so, then the assumption that arc plutons geochemically resemble frozen liquids is dubious and should be tested on a case-by-case basis. Moreover, comparisons of plutonic rock compositions with those of potentially comagmatic volcanic rocks will commonly yield spurious results unless accumulation in the plutons is accounted for.