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首页> 外文期刊>Bulletin of Volcanology >Endogenous growth in channelized komatiite lava flows: evidence from spinifex-textured sills at Pyke Hill and Serpentine Mountain, Western Abitibi Greenstone Belt, Northeastern Ontario, Canada
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Endogenous growth in channelized komatiite lava flows: evidence from spinifex-textured sills at Pyke Hill and Serpentine Mountain, Western Abitibi Greenstone Belt, Northeastern Ontario, Canada

机译：通道化科迈特岩熔岩流的内源性生长：加拿大安大略省东北部阿比比比绿石带西派克山和蛇纹石山的棘状纹理基岩证据

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摘要

Spinifex-textured sills (i.e., veins) characterized by komatiitic magmas that have intruded their own volcanic-piles have long been recognized. For instance, in the early 1970s, Pyke and coworkers, in their classic work at Pyke Hill in Munro Township, noted that not all spinifex-bearing ultramafic rocks formed as lava flows, rather some were clearly emplaced as small dikes and sills. Several hypotheses have been proposed to explain spinifex-textured sills: intrusion into a cold host, filter pressing, or drainage of residual liquid. However, these do not satisfactorily explain the phenomenon. Field and petrographic observations at Pyke Hill and Serpentine Mountain demonstrate that spinifex-bearing komatiite sills and dikes were emplaced during channel inflation processes when new magma was intruded into a cooler, semi-consolidated but permeable cumulate material. Komatiitic liquids were intruded into the olivine cumulate rocks near the boundary between the spinifex and the cumulate zones of well-organized to organized komatiite flows. Spinifex-textured sills are generally tabular in morphology, stacked one above another, with curviplanar contacts sub-parallel to stratigraphy. Some sills exhibit complex digitated apophyses. Thinner sills typically have a random olivine spinifex texture similar, though generally composed of coarser crystals, to that of komatiite lava flows. Thicker sills exhibit more complex organization of their constituent crystals characterized by zones of random olivine spinifex, overlying zones of organized coarse spinifex crystals similar to those found in lava flows. They have striking coarse dendritic spinifex zones composed of very large olivine crystals, up to several centimetres long and up to 1 cm wide that are not observed in lava flows. Typically, at the sill margins, the cumulate material of the host flow is composed of euhedral to subhedral olivine crystals that are larger than those distal to the contact. Many of these margin-crystals have either concentric overgrowth shells or dendritic olivine overgrowths that grew from the cumulate-sill contact toward the sill interior. The dendrites grew on pre-existing olivine cumulate at the contact in response to a sharp temperature gradient imposed by the intrusion of hot material, whereas the concentric overgrowths formed as new melt percolated into the unconsolidated groundmass of the host-flow cumulate material. Spinifex-textured sills and dikes occur in well-organized to organized flows that are interpreted to have formed by “breakouts” above and peripheral to lava pathways (channels/conduits) as a result of inflation that accompanied voluminous komatiitic eruptions responsible for the construction and channelization of komatiitic flow fields. The spinifex-textured dikes and sills represent komatiitic lava that was originally emplaced into the channel roof during periods of episodic inflation that resulted in lava breakouts and was subsequently trapped in the “roof rocks” during periods of channel deflation. Accordingly, the occurrence of spinifex-textured sills and dikes may indicate proximity to, and aid in the identification and delineation of lava channel-ways that could potentially host Ni–Cu–(PGE) mineralization within komatiitic lava flow-fields.
机译：长期以来，人们认识到具有以纺锤状岩浆为特征的刺状纹理的窗台（即静脉），这些岩浆侵入了自己的火山桩。例如，在1970年代初期，Pyke及其同事在Munro镇Pyke Hill的经典作品中指出，并非所有带有尖晶石的超镁铁质岩石都形成为熔岩流，而是明显地将其安置为小型堤坝和窗台。已经提出了几种假说来解释带有尖刺的窗台：侵入冷的主体，压滤或排出残留的液体。但是，这些不能令人满意地解释该现象。在派克山和蛇纹石山的野外和岩相观测表明，当新岩浆侵入较冷，半固结但可渗透的堆积材料中时，在通道膨胀过程中放置​​了含尖晶石的科马蒂岩基岩和堤坝。胶结岩液体被注入到尖晶石与组织良好的科马铁矿流的堆积区之间的边界附近的橄榄石堆积岩中。带有Spinifex质感的门槛通常呈板状，一个接一个地堆叠，其曲线平面接触与地层平行。一些窗台表现出复杂的数字化的附生菌。较薄的窗台通常具有类似于橄榄石熔岩流的随机橄榄石脊柱形纹理，尽管通常由较粗的晶体组成。较厚的基石表现出其组成晶体的更复杂的组织，其特征为随机的橄榄石棘齿形区域，与熔岩流中发现的那些相似的有组织的粗糙棘齿形晶体的覆盖区域。它们具有由非常大的橄榄石晶体组成的粗大的树突棘形区，其长达数厘米，宽达1厘米，在熔岩流中没有观察到。通常，在底边处，主体流的累积材料由比接触远端的那些大的从正方到方面的橄榄石晶体组成。这些边缘晶体中的许多晶体要么具有同心的过度生长壳，要么具有从累积的门槛接触向门槛内部生长的树枝状橄榄石过度增长。树枝状晶体在接触时已经存在的橄榄石堆积物上生长，这是由于热物质侵入而产生的急剧温度梯度所致，而当新的熔体渗入基质流动累积物质的未固结地基中时，同心的过度生长就形成了。 Spinifex纹理的窗台和堤坝以井井有条的形式流向井壁，这些井流被解释为熔岩通道（通道/管道）上方和外围的“爆发”所形成，这是由于伴随着大规模的火山喷发引起的通货膨胀Komatiitic流场的通道化。带有尖刺结构的堤坝和窗台代表的是科马蒂奇熔岩，该熔岩最初在偶发性膨胀时期进入通道顶，导致熔岩破裂，随后在通道收缩期间被困在“屋顶岩石”中。因此，带有尖刺结构的窗台和堤坝的出现可能表明与熔岩河道的接近，并有助于确定和描绘熔岩河道，这些熔岩河道可能潜在地导致了科马蒂奇熔岩流场内的Ni-Cu-（PGE）矿化。

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