Deep entrapment of buoyant magmas by orogenic tectonic stress: Its role in producing continental crust, adakites, and porphyry copper deposits
Deep entrapment of buoyant magmas by orogenic tectonic stress: Its role in producing continental crust, adakites, and porphyry copper deposits
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DOI：
10.1016/j.earscirev.2021.103744
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发表时间：
2021-09
影响因子：
12.1
通讯作者：
R. Loucks
中科院分区：
文献类型：
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作者：
R. Loucks
典型水化状态（≥2 wt% H2O）的弧玄武岩熔体相对于所有常见的地壳火成岩类型具有浮力，并且可以从地幔源通过大陆地壳直接绝热传递到浅层或火山口，而在运输过程中没有明显的结晶。玄武岩是大陆上最常见的喷发岩石。那么，为什么大陆地壳的平均整体成分是“安山岩”，而大陆地壳的底部又被垂直划分为“玄武岩”成分呢？在大陆边缘岩浆带非压缩段，喷发型和浅成型（隐晶质）岩体的频率模式为~50 wt% SiO2，全岩Sr/Y随SiO2的升高而下降。在大陆边缘岩浆带造山变形段，喷发岩和浅成岩的频率模式为~60 wt% SiO2， Sr/Y随SiO2的升高而升高。水平压缩通过岩脉扩展抑制浮力岩浆的上升。造山水平偏应力的典型值（~10 ~ 30 MPa）可超过推动岩脉扩展的岩浆浮力，并可在所有地壳深度圈闭数公里厚的浮力岩浆。在造山过程中，水平压应力增大，岩脉扩展的阻力增大，因此在更深的地壳层次上实现了更高级的化学分异。弧莫霍处流体不饱和的围岩温度通常高于岩浆的湿固相温度，因此残余的长硅熔体不能通过向围岩导热散失而冻结。热“不朽”的残余花岗岩岩浆可能会被储存在莫霍附近的应力圈闭中，只要强大的造山压应力持续5-10万年或更长时间，并经历地幔玄武岩熔融的多次补充。在莫霍附近产生的超镁铁质晶体堆积在地震上与区域地幔无法区分，并被包括在地幔成分清单中，从而在地震莫霍之上留下了有偏差的大陆地壳成分清单。弧段的火山-深部质量比是构造应力状态的敏感函数。岩浆跨地壳输送速率不应被误解为弧岩浆产生速率。上地壳中花岗岩类岩浆活动的“高通量”脉冲往往大大滞后于地幔中岩浆生产速率的板块运动学指标。当造山压应力从峰值减弱时，上地壳的高通量花岗岩类岩浆活动就会发生，这使得浮力岩浆从较深地壳（或最上层地幔）的应力圈闭中长期储存出来。“过增厚”造山带的造山崩塌始于横向无约束的最高海拔，并缓慢向下传播，依次进入具有较高Sr/Y的应力圈闭岩浆储层。60-70 wt% sio2的全岩Sr/Y比地壳厚度更能反映构造应力。在莫霍盆地附近间歇性补充应力圈闭的分块结晶过程中，热不朽的残余花岗岩熔体演化为高（“埃达基”）Sr/Y和溶解的H2O、SO3和Cl的特殊继承，当这些熔体在造山应力开始从莫霍盆地附近深度的峰值减弱时逸出到浅深度时，这些熔体具有岩浆-热液成矿能力。通过对火成岩成因的认识，改进了寻找斑岩型铜矿床的策略。
Arc-basaltic melts of typical hydration state (≥ 2 wt% H2O) are buoyant relative to all common crustal igneous rock types and may pass adiabatically from a mantle source through continental crust directly to shallow depths or volcanic vents without significant crystallisation in transit. Basalts are the most common eruptive rocks on continents. So why is the average bulk composition of continental crust “andesitic”, and why is continental crust vertically zoned with “basaltic” compositions at the bottom? In non-compressive segments of continent-margin magmatic belts, the frequency mode for eruptive and hypabyssal (aphanitic groundmass) rocks is ~50 wt% SiO2, and whole-rock Sr/Y falls with rising SiO2. In orogenically deforming segments of continent-margin magmatic belts, the frequency mode of eruptive and hypabyssal rocks is ~60 wt% SiO2, and Sr/Y rises with rising SiO2. Horizontal compression inhibits ascent of buoyant magmas by dyke propagation. Typical values of orogenic horizontal deviatoric stress (~10–30 MPa) can exceed the magma buoyancy force driving dyke propagation and can trap kilometres-thick sills of buoyant magma at all crustal depths. As horizontal compressive stress increases during orogeny, resistance to dyke propagation increases, so more advanced chemical differentiation is achieved at deeper crustal levels. The temperature of fluid-undersaturated country rock at the arc Moho typically is higher than the magma's wet solidus temperature, so residual felsic melts cannot freeze by conductive heat loss to country rocks. Thermally “immortal” residual granitoid magmas may remain stored in Moho-vicinity stress traps as long as strong orogenic compressive stress lasts—5-10 million years or more—and experience multiple replenishments by basaltic melt from the mantle. Ultramafic crystal cumulates produced near the Moho are seismically indistinguishable from regional mantle and are included in the mantle composition inventory, leaving a biased composition inventory of continental crust above the seismic Moho. The volcanic-to-plutonic mass ratio in arc segments is a sensitive function of the tectonic stress regime. Rates of trans-crustal magma transmission should not be misinterpreted as arc magma production rates. “High-flux” pulses of granitoid magmatism in the upper crust tend to substantially lag plate-kinematic indicators of magma production rates in the mantle. Episodes of high-flux granitoid magmatism in the upper crust occur as orogenic compressive stress wanes from peak values, permitting escape of buoyant magmas from long-term storage in stress traps in the deeper crust (or uppermost mantle). Orogenic collapse in “over-thickened” orogens begins at the laterally unconfined highest elevations, and propagates slowly downward, successively tapping stress-trapped magma reservoirs having higher Sr/Y. Whole-rock Sr/Y at 60–70 wt% SiO2is a better proxy for tectonic stress than for crustal thickness. During fractional crystallisation in intermittently replenished stress traps near the Moho, thermally immortal residual granitoid melts evolve to high (“adakitic”) Sr/Y and exceptional inheritances of dissolved H2O, SO3, and Cl, which endow them with magmatic-hydrothermal copper-ore-forming capability when such melts escape to shallow depths as orogenic stress begins to wane from peak values at Moho-vicinity depths. Improved strategies in the search for porphyry copper ore deposits emerge from understanding the igneous petrogenesis.