Two-Stage, Extreme Albitization of A-type Granites from Rajasthan, NW India
Two-Stage, Extreme Albitization of A-type Granites from Rajasthan, NW India
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DOI：
10.1093/petrology/egs003
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发表时间：
2012-05-01
影响因子：
3.9
通讯作者：
Baumgartner, Lukas P.
中科院分区：
文献类型：
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作者：
Kaur, Parampreet;Chaudhri, Naveen;Baumgartner, Lukas P.
钠长石化是一种常见的过程，在此过程中，热液流体将斜长石和/或钾长石转化为几乎纯的钠长石；然而，其在花岗岩类中的具体机制尚不清楚。在印度拉贾斯坦邦Khetri杂岩中，约1700 Ma a型铝质铁质花岗岩在很大程度上受到两个交代锋面的影响，即最初的寡长石向近纯钠长石的转变和随后的微斜长石被微斜长石所取代，微斜长石带与无微斜长石带之间有明显的接触。钠长石作用使原来的粉灰色花岗岩漂白，变成白色。矿物学变化包括转变奥长石(类似于一个(12))和微斜长石(类似于或(95))几乎纯钠长石(类似于一个中心(0点5 - 2),角闪石从potassian ferropargasite (X-Fe 0中心84 - 0点中心86点),钾的绿钠闪石(X-Fe 0中心88 - 0点中心点97)和阳起石(X-Fe 0中心导32-0中心导67),和黑云母铁云母(X-Fe 0中心71 - 0点中心74点),铁云母(X-Fe 0中心90 - 0点中心91点)。全岩等长线图显示，在钠长岩化过程中，花岗岩主要发生水化作用，Si含量小幅增加，Na含量大幅增加，而K、Mg、Fe、Ca、Rb、Ba、Sr、Zn、轻稀土元素和u均损失，全岩Sm-Nd同位素数据在1419 +/- 98 Ma的等长线上显示出明显的干扰和侵入年龄的至少部分重置。全岩Rb- sr等时线的严重散射反映了完全钠长石化样品中Rb的极度损失，有效地将钠长石花岗岩中的Sr-87/Sr-86比值冻结在很高的值（0中心点725-0中心点735）。这表明，要么是高放射性成因的锶从乡村岩石中渗入，要么更有可能是在原始侵入和钠长石化之间相当长的一段时间内（估计至少为300迈）放射性成因的向内生长。钠长石化发生在350 ~ 400℃，这是由上升的热液渗透造成的，热液在更低温度下通过变质岩运移，获得了较高的Na/K和Na/Ca比值。氧同位素比值从原始花岗岩的δ O-18 = 7‰增加到完全石化样品的9-10‰，表明流体已与周围变质地壳平衡。利用流体渗透色谱理论的交代模型解释了产生所观察到的分带的过程，交代前沿是原始花岗岩的低晶长石转化为钠长石，其次是微斜长石也转化为钠长石的尾随锋。驱动流体渗透的温度梯度可能是由花岗岩本身的高发热量产生的。沿北东-西向Khetri线发育的钠长花岗岩的局限和钠长岩化的普遍性表明，钠长岩化流体可能起源于该线的活化过程。更一般地说，a型花岗岩内部高产热引起的稳态温度梯度可能为其他高富集花岗岩类岩体类似的交代成矿过程提供动力。
Albitization is a common process during which hydrothermal fluids convert plagioclase and/or K-feldspar into nearly pure albite; however, its specific mechanism in granitoids is not well understood. The c. 1700 Ma A-type metaluminous ferroan granites in the Khetri complex of Rajasthan, NW India, have been albitized to a large extent by two metasomatic fronts, an initial transformation of oligoclase to nearly pure albite and a subsequent replacement of microcline by albite, with sharp contacts between the microcline-bearing and microcline-free zones. Albitization has bleached the original pinkish grey granite and turned it white. The mineralogical changes include transformation of oligoclase (similar to An(12)) and microcline (similar to Or(95)) to almost pure albite (similar to An(0 center dot 5-2)), amphibole from potassian ferropargasite (X-Fe 0 center dot 84-0 center dot 86) to potassic hastingsite (X-Fe 0 center dot 88-0 center dot 97) and actinolite (X-Fe 0 center dot 32-0 center dot 67), and biotite from annite (X-Fe 0 center dot 71-0 center dot 74) to annite (X-Fe 0 center dot 90-0 center dot 91). Whole-rock isocon diagrams show that, during albitization, the granites experienced major hydration, slight gain in Si and major gain in Na, whereas K, Mg, Fe and Ca were lost along with Rb, Ba, Sr, Zn, light rare earth elements and U. Whole-rock Sm-Nd isotope data plot on an apparent isochron of 1419 +/- 98 Ma and reveal significant disturbance and at least partial resetting of the intrusion age. Severe scatter in the whole-rock Rb-Sr isochron plot reflects the extreme Rb loss in the completely albitized samples, effectively freezing Sr-87/Sr-86 ratios in the albite granites at very high values (0 center dot 725-0 center dot 735). This indicates either infiltration of highly radiogenic Sr from the country rock or, more likely, radiogenic ingrowth during a considerable time lag (estimated to be at least 300 Myr) between original intrusion and albitization. The albitization took place at similar to 350-400 degrees C. It was caused by the infiltration of an ascending hydrothermal fluid that had acquired high Na/K and Na/Ca ratios during migration through metamorphic rocks at even lower temperatures in the periphery of the plutons. Oxygen isotope ratios increase from delta O-18 = 7 parts per thousand in the original granite to values of 9-10 parts per thousand in completely albitized samples, suggesting that the fluid had equilibrated with surrounding metamorphosed crust. A metasomatic model, using chromatographic theory of fluid infiltration, explains the process for generating the observed zonation in terms of a leading metasomatic front where oligoclase of the original granite is converted to albite, and a second, trailing front where microcline is also converted to albite. The temperature gradients driving the fluid infiltration may have been produced by the high heat production of the granites themselves. The confinement of the albitized granites along the NE-SW-trending Khetri lineament and the pervasive nature of the albitization suggest that the albitizing fluids possibly originated during reactivation of the lineament. More generally, steady-state temperature gradients induced by the high internal heat production of A-type granites may provide the driving force for similar metasomatic and ore-forming processes in other highly enriched granitoid bodies.