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Peritectic Horblende: Water-Assisted Melting of Calc-Alkaline Intermediate to Mafic Rocks, Ladakh, NW India

Roberto Weinberg, Monash University, Australia



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This page shows distinct features related to the partial melting of two outcrops in the Muglib pluton, close to the village of Tangtse in Ladakh, NW India. Tng60  N34o 03' 25.3" E78o 14' 13.3":


NOTES FOR AUTHOR:  a) new Hbl forming on old and new in leucos 779, c) mottled pattern indicative of melt extraction leaving behind large new Hbln ph 780-781 788-789 (excellent channelway), ph 793 show beautiful gradation from mottled to residual large hbl in leucocratic groundmass, to leucogranite escap, d) hbl aggregate 771, e) boudinage 797, f) typical mobile leucosome 768
Tng165 N34o 03' 08.7" E78o 14' 17.7"


NOTES FOR AUTHOR: a) leucosome with melanosome rim ph 739 or 744  (feature not available for Tng60 because melting took place within pores in the rock as evidenced by mottled pattern, and not along sheets),  b) 750 and 747, 753zoom melt extraction from pores to dyks with newly formed hbl porphyroblasts, c) 757 huge hbl porphyroblast


Tng 165
anatexis diorite hornblende porphyroblasts in leucogranite
Figure 1a. (753)  In situ melting pattern in diorite. Leucocratic dykes have irregular boundaries against the diorite, and merge with irregular leucocratic patches in the matrix. Figure 1b. (750)  Detail of a. Note hornblende porphyroblasts in leucogranite.

Figure 1c.  (747). Leucogranite with hornblende porphyroblasts, and cross-cutting leucosome in country rock linked up continuously with layer parallel leucosomes.


hornblende porphyroblasts in leucogranite hornblende porphyroblasts in leucogranite
Figure 2a (744).  Contact between diorite and leucogranite. Porphyroblasts of hornblende grow preferentially at the irregular contacts. Figure 2b (739) . Growth of porphyroblasts along the contact between leucogranite and diorite. The central wedge-shaped aggregate is presumed to have grown over a pre-existing wedge of diorite isolated by leucogranite.
hornblende porphyroblasts in leucogranite
Figure 2c (757). Porphyroblasts of hornblende in leucogranite. Porphyroblasts seem to grow inward from the margin of the leucogranite, which here runs horizontally at the top of the photograph.


Interpretation of Tng165. In situ melting formed a leucogranite and the new growth of hornblende porphyroblasts as a peritectic phase. This growth took place preferentially along contact leucogranite-diorite and produced grains larger than 2cm across. The question is how did water gain acess to the rock? Leucogranite in this outcrop tend to parallel foliation. Could it be that they replicate or mark the water inflow path? DId the inlfow path then become the magma outflow path?


Tng 60
Mottled diorite
Figure 2a. (781).  Mottled diorite. Patches of medium-grained diorite ar3e surrounded by regions comprised of leucocratic material around porphyroblastic hornblende. Figure 3b  (789).
Mottled diorite
Figure 3b. (788). The mottled diorite has relatively little interstitial leucocratic material that here comprises a diagornal band across the rock becoming wider towards the top right-hand-side. Note the coarse hornblende throughout the rocks. Interpretation: leucocratic material represents the remainder of a melt fraction that escaped the interstices of the mottled rock to form a channel, leaving behind the newly grown, peritectic hornblende in the interstices.


Porphyroblasts of hornblende in migmatite
Figure 4a. (779).  Contact between diorite and leucogranite. Porphyroblasts of hornblende grow at the irregular contacts and are also found in the leucogranite. Figure 4b.  (797) .
Figure 4c. (793). Staining in orange is due to lichen.


Aggregates of hornblende porphyroblasts in leucogranite schlieren of hornblende
Figure 5a  (771) . Aggregates of hornblende porphyroblasts at the contact between leucogranite and the paleosome. Figure. 5b. (768). Typical leucogranites in the vicinity of the mottled diorite, characterized by schlieren of hornblende.


Interpretation of Tng60. Melting of diorite produced a leucogranite and peritectic hornblende. When preserved in the matrix the process produces the mottled diorite texture. Melt extraction from the interstices left hornblende porphyroblasts behind (Fig. 3). Hornblende grains are also extracted as solids with the melt but tend to separate further as aggregates at the contacts with the source rock (Fig. 5a) or form schlieren (Fig. 5b).