Metamorphism of an obducted island arc: Example of the Kohistan sequence (Pakistan) in the Himalayan collided range

Earth and Planetary Science Letters, 65 (1983) 133-144 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

133

[51

Metamorphism of an obducted island arc" example of the Kohistan sequence (Pakistan) in the Himalayan collided range Jean Pierre Bard U.S.T.L., Laboratoire de Pdtrologie des Zones Profondes, F-34060 Montpellier Cedex (France)

Received January 3, 1983 Revised version accepted May 5, 1983

In northern Pakistan, the extraordinary 40-km-thick Kohistan sequence of metamorphosed mafic, ultramafic and calc-alkaline layered plutonic and volcanic (mainly andesitic to rhyodacitic) rocks has been recently reinterpreted as the only complete vertical section of an intra-oceanic island arc presently exposed anywhere in the world. Plate scale-models have been suggested to explain the origin and the tectonic evolution of this arc in the Himalayan collision range. Despite some noticeable differences in the models, there is general agreement that the Kohistan sequence represents the crust of an arc obducted onto the northern edge of the Indian plate before the Oligocene collision of India against Asia. New data on the tectonometamorphic evolution of the Kohistan sequence have led to the conclusion that a first major D 1 event developed with increasing metamorphic grades oriented downward the Kohistan pile but also toward "ensandwiched" basic-ultrabasic granulitic rocks. Rare geochronological data and petrological evidence indicate that the latter granulites (pyriclasites and various plagiopyrigarnites, pyrigarnites, metatroctolites, metawebsterites, metadunites, etc.) were parts of enormous Lower to Middle Cretaceous layered calc-alkaline plutons emplaced during the arc-building stages. As the D 1 metamorphic event is correlated with the Upper Cretaceous obduction process of the Kohistan onto India, it is proposed that the D 1 thermal structure was strongly controlled by remnant magmatic heat source(s) within the obducted arc. This interpretation fits nicely with recent theoretical thermal models since the obduction of the arc was probably shortly after (10-20 Ma) the emplacement of the former plutonic arc rocks. A blueschist "tectonic m+lange" underlying the obducted arc was possibly synchronous to the obduction and not clearly linked to the older subductive process. The Oligocene collision of India against Asia was associated with a Barrovian overprinting metamorphism in the Kohistan arc. During this second event the dynamo-thermal structure was much more classical and not controlled by the occurrence of the Cretaceous metaplutonic rocks. A tentative model of the thermal structures during the major events which affected the Kohistan arc is proposed.

1. Geological setting and metamorphic zonation of the Kohistan area Preliminary structural, textural and mineralogical studies [1-4] of the extraordinary Kohistan " a r c s e q u e n c e " [5,6] i n d i c a t e t h a t t w o m a i n t e c t o n o m e t a m o r p h i c e v e n t s ( D 1 a n d D 2 ) w e r e responsible for the actual metamorphic structure of the northern Pakistan units ensandwiched by the M a i n M a n t l e T h r u s t ( M M T [5]), t h e N o r t h e r n M e g a s h e a r z o n e ( = N o r t h e r n S u t u r e [5]) a n d t h e N a n g a - P a r b a t a n t i f o r m (Fig. 1A). T h e D 1 m e t a m o r p h i c z o n a t i o n is t h e m a j o r o v e r p r i n t . I t h a s 0012-821x/83/$03.00

© 1983 Elsevier Science Publishers B.V.

been studied using textural and field distribution of early minerals either in strongly schistose a m p h i b o l i t i c u n i t s o r i n m o r e o r less f o l i a t e d a n d sheared two-pyroxene and garnet-clinopyroxene granulitic complexes (respectively a wide 8-10 km thick calc-alkaline layered metanoritic lopolith c a l l e d t h e " g r a n u l i t i c b e l t " [1] a n d a d e e p - s e a t e d 5-6 km thick mafic-ultramafic formation mainly built up by pyrigarnites, diopsidites, garnetites and m e t a d u n i t e s c a l l e d t h e " J i j a l - P a t a n c o m p l e x " [1] and equivalents near Mingora). The conclusions regarding the Dl-tectonometamorphic events may b e s u m m a r i s e d as f o l l o w s :

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Fig. lB. Simplified cross-section of the Kohistan from Jijal (J) to Gilgit (G). 1 = layered metaplutonic rocks, i.e. the Jijal-Patan (P) complex, the main granulitic belt and some associated small lopoliths; 2 = undifferentiated amphibolitic series; 3 = Kalam metasediments (greywackes, shales, cherts and marbles) with pillowed metabasalts; 4 = Utror (U) calc-alkaline volcanics (mainly andesites) and associated metasediments (greywackes and shales); 5 = Yasin sedimentary group (mid-Cretaceous) with basal conglomerates; 6 = Indian plate metamorphics; 7 = Asia metamorphics; MMT = Main Mantle Thrust; NS = Northern Suture (for simplicity, the numerous syn- to late-D2 dioritic intrusives (see map, Fig. 1A) have not been drawn); DMZ = ductile late-D 1 Main Shear Zone.

(1) The syn- to late-D1 metamorphic stages led to the development of " w e t " and " d r y " metamorphic rocks, i.e., respectively: - - m e t a s e d i m e n t a r y (cherts, shales, greywackes and basaltic tuffites, calcareous lenses), metavolcanic (mainly basaltic (tholeiitic) to andesitic) and "retrogranulitic" rocks (i.e., amphibolite facies amphibolitized metanorites, metagabbros, metatroctolites and basic blastomylonites) bordering or cutting across the granulitic belt and the JijalP a t a n / N o r t h Mingora complexes; - - m e t a b a s i c , metamafic and meta-ultramafic layered rocks of plutonic origin now transformed into pyriclasites (pyroxene-plagioclase granulites) and various plagiopyrigarnites (plagioclase + garnet + clinopyroxene _+ orthopyroxene-bearing granulites), pyrigarnites, metawebsterites, metatroctolites, metadunites and metaharzburgitic rocks. (2) These rocks show syn- to late-D 1 critical minerals a n d / o r metamorphic assemblages which seem near-equilibrium within the following metamorphic zones (Fig. 2): - - a "chlorite+ epidote" low-grade zone which outcrops in the northernmost parts of Kohistan, within the so-called Utror metavolcanic (andesitic to rhyodacitic [7]) synform and, possibly, South Chilas in the Thak Valley;

--an "'actinolite/blue-green hornblende + clinozoisite-epidote" zone with biotite + muscovite + (scarce) andalusite aluminous layers;

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staurolite + c o r u n d u m _+ garnet + plagioclasebearing associations either in leucocratic layers, late-D 1 pegmatoid metatrondhjemitic injections or within late-D t fractures affecting various retrogranulitic rocks; - - a " b a s i c migmatic" zone with in-situ or near-situ syn- to late-D 1 garnet-bearing leucosomes at the expense either of retrogranulitic rocks or striped amphibolites; hypersthene + garnet + biotite associations also d e v e l o p in s o m e metagreywacke rocks which also contain calc-silicate-bearing "skarns"; - - a " t w o - p y r o x e n e granulitic" zone covering a wide range of rocks of magmatic origin; a m o n g them layered (clearly cumulative) metatroctolites showing a "plagioclase + olivine (FOB0) out" reactional isograd together with late-D 1 (not magmatic) amphibolitization (Mg-pargasite growth in orthopyroxene-clinopyroxene + green spinel coronas around olivine). It is noteworthy that the equilibrated clinopyroxene-orthopyroxene pairs of this granulitic zone are chemically similar [2-4] to

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Fig. 2. S c h e m a t i c m e t a m o r p h i c z o n a t i o n related to the D l / l a t e - D l t e c t o n o m e t a m o r p h i c stages in K o h i s t a n . l = J i j a l - P a t a n c o m p l e x ; 2 = a m p h i b o l i t e units; 3 = g r a n u l i t e belt; 4 = U t r o r v o l c a n i c s ; 4 ' = m e t a s e d i m e n t s : 5 = Y a s i n g r o u p ( M i d d l e C r e t a c e o u s ) ; 6 = dioriticg r a n o d i o r i t i c syn- to p o s t - D 2 intrusives; 7 = b l u e s c h i s t belt a n d a s s o c i a t e d " t e c t o n i c m ~ l a n g e " ; 8 - 9 = a c t i n o l i t e / b l u e g r e e n h o r n b l e n d e + e p i d o t e zone; 9 - 1 0 = g a r n e t + g r e e n h o r n b l e n d e + clinozoisite zone; 1 0 - 1 1 = b a s i c m i g m a t i c zone: 11 - 13 = twop y r o x e n e zone; 12 = olivine + p l a g i o c l a s e " o u t " in troetolitic rocks; 13 = g a r n e t + c l i n o p y r o x e n e _+clinozoisite " i n " ; 14 = g a r n e t " i n " in l a t e - D 2 l e u c o c r a t i c injections c r o s s c u t t i n g the g r a n u l i t i c rocks; 15 = syn- to l a t e - D 1 h i g h - g r a d e a m p h i b o l i t e facies a m p h i b o l i t i z a t i o n o f g r a n u l i t i c r o c k s ( " r e t r o g r a n u l i t i c z o n e " ) ( a r r o w for i n c r e a s i n g m e t a m o r p h i c grade)•

the 7 5 0 - 8 5 0 ° C / 6 - 1 0 kbar pairs or various catametamorphic complexes from old basements. - - a "garnet + clinopyroxene + clinozoisite" zone corresponding to the syn- to late-D 1 destabilisation of early orthopyroxene + clinopyroxene + Mg-pargasitic hornblende. The latter are rather from previously formed two-pyroxene foliated granulose gneisses (various pyriclasites from metaleuconoritic, noritic and troctolitic rocks) than from pre-D t mafic-ultramafic layered plutonic rocks (the Jijal-Patan/North Mingora complexes). Garnet-clinopyroxene pairs from the plagiopyrigarnites and pyrigarnites which develop in this zone yield around 850+ 5 0 ° C / 1 2 - 1 3 kbar depending on the nature of the rocks (plagioclase-free or plagioclase-bearing) and over all, the geother-

mobarometry equations used. Such estimates are higher than those from the two-pyroxene granulitic belt (800 + 5 0 ° C / 9 + 2 kbar) [4]. (3) the superimposed D 2 tectonometamorphic event is clearly visible on the rocks from the previously described metamorphic zones but it is not easy to draw a detailed metamorphic zonation for this second folding (and syn-schistosity S2phase). Grossly, D 2 is associated with greenschist to epidote-amphibolite facies retrogressions. The latter metamorphic facies seems rather well developed in the lower part of the Kohistan sequence, i.e., at the expense of the amphibolite units and the granulitic belt and complexes. Currently D 2 is well expressed in the numerous ductile shear zones exposed toward the basis of the Kohistan pile.

137 These ductile zones are mylonitic b a n d s with southwards displacement. They are different in time, place a n d t e m p e r a t u r e estimates of deformation from early late-D a, n o r t h w a r d s m o v i n g shear zones; the latter are spectacular meter-to-hectometer-thick b l a s t o m y l o n i t i c b a n d s t h r o u g h o u t the a m p h i b o l i t e units and, over all, the lowermost parts of the granulitic belt a n d the u p p e r m o s t parts of the Jijal-Patan complex ( D M Z [4]; Fig. 1B).

2. Summary of the metamorphic paths and the estimated metamorphic gradient The P , T paths d u r i n g the m e t a m o r p h i c stages of the K o h i s t a n sequence have been tentatively d r a w n in Fig. 3. As deduced from the isograd pattern, the m e t a m o r p h i c z o n i n g a n d the thermob a r o m e t r y data [3,4], the estimated T-depth curves d u r i n g D 1 (curves A, A' a n d A") a n d D 2 (possible curve B) lead to the following c o m m e n t s : (1) The presence of andalusite in rare silicoa l u m i n o u s beds indicates that the D ~ / l a t e - D 2 average m e t a m o r p h i c gradient ("geotherms-" A, A' a n d A") was p r o b a b l y near 3 0 ° C / k m in the part of the sequence which suffered the epidote-amphibolite to a m p h i b o l i t e facies conditions. Geot h e r m o m e t r y estimates as well as the sharp transition of a zone of pegmatoid metatects a n d the destruction of olivine Fo80 in the presence of Caplagioclase strongly suggest that the D~-"geot h e r m " was >7 8 0 - 1 0 0 ° C / k m close to a n d inside the borders of the granulite unit in the range 7 5 0 - 8 5 0 ° C . W i t h i n the latter, P (estimated by the actual overlying series) was at least 6 - 8 k b a r a n d T estimates would indicate no significant temperature elevation with depth. I n such a situation it is proposed that the dT/dPIoad decreases towards the a p p a r e n t b o t t o m of the granulite u n i t to become negatively 80 _+ 5 ° C / k m a n d progressively r e t u r n to a positive slope 25 + 5 ° C / k m in the u n d e r l y i n g rocks ( a m p h i b o l i t i z e d " r e t r o g r a n u l i t e s " ) c o n t a i n i n g late-D~ i n j e c t i o n s with kyanite-staurolite associations a n d garnet-bearing, sometimes migmatitic in aspect, amphibolites from the so-called southern K o h i s t a n a m p h i b o l i t e unit. T h e occurrence of andalusite a n d kyanite with

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Fig. 3. Tentative P(Pioad)-T paths during the metamorphic evolution of the Kohistan island arc. "Geotherms" A and A' respectively for earlier and k'ate-D~event when cross-cutting the granulitic belt (metanoritic layered intrusive) and the Jijal-Patan complex (for A' see also Fig. 4B); "geotherm" A" for syn- to late-D l event for the latter complexes (see also Fig. 4B); "geotherm" B for syn- to late-DEevent (K-S-A: stability fields of the AIESiO5 polymorphs); 1 = beginning of partial melting of water-saturated gabbros [8]; 1'= approximate melting curve of amphibolitic rocks [38]; 2 = zo + ky + quartz ~ H zO + L [39]; 3 = an + HEO (low-P association)~ zo + ky + quartz (high-P association) [39]; 4, 4'= pure Mg-staurolite high-P stability field [46-48]; 5 = an+zo+quartz_+gt+ H20 ~ L [39]; 6 = various stability fields in the system CaO-MgO-A1203-SiO2HEO ([2]: an+forst+HEO, [~]: an+hb+cpx+sp+HEO, [ ] : an + cpx + opx + sp + HEO); 7 = approximate reaction field of ol + an ~ cpx + opx + sp in tholeiitic basahs (see literature in references 40, 41, 42); 8 = approximate reaction field of opx + an ~ cpx + gt in tholeiitic basahs [40,49,50]; 9 = Al-ep ~ zoisite [43]; 10 = epidote + q ~ gt + an + mgt + HEO ([44[; NNO buffer); 11 = chlorite "out" in metabasites [45] as isograd separating the greenschist facies from the so-called epidoteamphibolite facies; 12 = epidote "out" from reaction ep+ An0_20 ~-~An30_40+q+HEO under HM buffer conditions (high foe conditions [45]). Circles with arrows: possible subin-situ melting of amphibolitic rocks giving late-D I uprising ky+zo+st (a) and/or gt+zo trondhjemitic melts (b) and dioritic sub-autochthoneous intrusives (c).

quite the same relative age respectively upon a n d under the granulite belt as well as the P, T esti-

138

mates on the gabbro + H 2 0 partial melting conditions [8] are in agreement with the drawn aspect of possible early (A) and late (A') P - T curves during D 1. Because of the need to take into account the T estimates in the granulite unit, the conclusion is that the Dl-"geotherms" are affected by a loop turned toward the T-axis in the range 6-80 kbar (17-22 km depth). Such a feature is not evident for the P - T curve in the western area (Tal area) of Kohistan where no granulites seem to occur. Here,

INTRAOCEANIC

the metamorphic curve would show a classical morphology (curve A"), i.e., it is similar to the "geotherms" in conductive models of heat transfers from deep parts. The aspects of the P - T curves A and A' towards the present deepest parts of the metamorphic pile are not well established. The P - T estimates and textural evidence in the Jijal-Patan complex would also indicate a second sharp drop of the temperature and a shifting of the P - T curve towards the

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Fig. 4. Possible aspects of the thermal structures in the Kohistan obducted island arc during its tectonometamorphic evolution. A. lntraoceanic subduction stage with arc building -- 100 Ma ago, i.e. just after the last emplacement of calc-alkaline plutonic and volcanic

rocks. 1 = hypothetic ascending upper mantle diapir [4] under the arc; 2 = partial melting of the diapir giving rise to the Jijal-Patan-type cumulative [1,4,21] mafic-ultramafic rocks and to the layered lopolithic intrusion (which will be the future granulite belt of Kohistan); 3 = basaltic tufts and lavas (tholeiitic, with abyssal a n d / o r arc affinities, up to calc-alkaline [1,51]; 4 = Utror andesites, dacites and rhyodacites (calc-alkaline series [7]) (the ocean relict between the Kohistan arc and India was probably (?) [15] 800 km wide 100 Ma ago; Asia was at 6500-6000 km from the arc). B. Obduction stage -- 85 Ma ago and D I event. A' and A" = vertical profiles for "geotherms" A' and A" in Fig. 3; 1 = granulite unit from a metamorphic calc-alkaline layered pluton; 2 = Jijal-Patan complex as remnant of an ascending mantle diapir which suffered partial melting; 3 = basic anatexis around the granulite and amphibolitization of the latter in response to H20 transfers from the neighbouring amphibolite units and associated metasediments; o.c = oceanic crust as the basement of the Kohistan arc sequence (note the high-pressure metamorphism in the blueschist belt under M M T is related to D1); M M T = Main Mantle Thrust; D M Z = ductile northwards main zone (approximate attitude). C. Oligocene collision of lndia (northwestern edge) against Asia during the D 2 tectonometamorphic event. Note here that the 50-40 Ma syn- to late-D 2 intrusives are not related to a Late Eocene/Oligocene southwards subduction on Andean type margin [10,18] but rather correlated (not proved) to a partial anatectic process (with possible hybridization with products from melting of the arc crust + underlying mantle) of the Indian plate under the obducted Kohistan arc.

T-axis. T h i s d r o p m i g h t h a v e b e e n m o r e p r o n o u n c e d d u r i n g t h e e a r l i e r ( o r p r e - ?) D a - s t a g e ( c u r v e A ) , a f e a t u r e w h i c h m a y e x p l a i n : (a) s o m e r e l i c t t w o - p y r o x e n e _+ p a r g a s i t e a s s o c i a t i o n s i n t h e J i j a l - P a t a n p l a g i o p y r i g a r n i t e s ; ( b ) t h e l a t e - D 1 orthopyroxene + plagioclase "out" reaction in some o f t h e s e r o c k s , a n d (c) s o m e r e l i c t (?) a n d p r o b l e m a t i c a l h i g h t e m p e r a t u r e s (>~ 9 0 0 ° C ) e s t i m a t e d in some metawebsterites and two pyroxene-spinel pyroxenites. M i n e r a l o g i c a l d a t a o n t h e s y n - to l a t e D 2 m e t a m o r p h i c e v e n t [4] a l l o w t h e d r a w i n g o f t h e P - T curve aspect during D2--no s i g n i f i c a n t dis-

turbances correlated either with the granulite belt o r t h e J i j a l - P a t a n c o m p l e x s e e m to o c c u r a t t h i s t i m e a n d it is s u g g e s t e d t h a t t h e " g e o t h e r m " h a s a slope 25-20°C/km in the middle and lower part of the Kohistan sequence.

3. G e o c h r o n o l o g i c a l data on D t and D 2 e v e n t s The great part of the Kohistan sequence was b u i l t [5,6] b e f o r e t h e A l b i a n t i m e s ( - - 100 M a ) , i.e. b e f o r e t h e d e p o s i t s o f t h e u n c o n f o r m a b l e fossilife r o u s " Y a s i n d e t r i t a l s e r i e s " [9]. B e c a u s e o f p o s s i -

140 ble Eocene microfauna around Dir and Late Cretaceous limestones in the Deosai Plateau, east of Kohistan [10], it is suggested that the Kohistan volcanic arc [4,5] was covered by clastic deposits of these ages before or during the first (D1) tectonometamorphic event. This latter develops around 85 Ma (i.e., Upper Cretaceous) with the following rare data: (1) 80 Ma on zircons of the Swat pyroxene granulites ( U / P b method) [11] without clear information whether this age is metamorphic or igneous. (2) 75 + 5 Ma on phengite from the blueschist belt underlying the thrusted arc near Shangla Pass ( 39mr//4°Ar method) [12,16]. (3) 82 + Ma on a hornblende from a "syenitic" rock intruding the metamorphosed Dras volcanics ( = Utror andesitic series) at Kargil in LadakhDeosai, east Kohistan (39Ar/a°mr method) [10]. (4) 67 Ma on a hornblende from a deformed pegmatite cutting the S~ foliation of the granulite belt around Barheim (Swat Valley) (39Ar/a°Ar method) [13]. These data point to a major Upper Cretaceous event (Da) in Kohistan that would be synchronous with the blueschist metamorphism in the M M T suture (Fig. 4B). It is suggested on the other hand that the calc-alkaline volcanism and plutonism in Kohistan are probably L o w e r / M i d d l e Cretaceous (110 + 10 Ma) with some late intrusions, perhaps Late Cretaceous (100 + 10 Ma). Similar ages have recently been given in Ladakh [14] that would correspond to cooling of noritic rocks similar to the granulitic belt in Kohistan. In the Yasin area (Fig. 1A) measurements of syn- to late-D 2 dioritic intrusions have synkinematic cooling between 56 and 37 Ma [5]. The 39Ar/4°Ar method has given 50 + 5 Ma ages on biotite (H. Maluski, in preparation) from late kinematic (D2(?)) diorites, east Deosai Plateau, and on biotite from metamorphic orthogneisses (Bunner orthogneiss) and surrounding metasediments within the Indian plate series immediately south of Kohistan [12]. The same method [10] yields an age of about 42 Ma from m e t a m o r p h i c micas in a d e f o r m e d (D 2) granodiorite north of the MMT suture near Skardu (Deosai-Ladakh Plateau, east of the Kohistan arc). All the measurements would indicate that D 2 is

probably Upper E o c e n e / L o w e r Oligocene in Kohistan-Ladakh. Taking into account a recent discussion about the wandering of India [15], it seems, together with earlier conclusions [16,17], that D 2 is synchronous with the collision of the northwestern edge of the India plate with Asia and synchronous with the first Alpine phase, with Barrovian metamorphism type [16], within the India plate south of the M M T suture. Considering the changing opening rates of the Indian Ocean [15,17] during the Mesozoic, it that seems the slow Lower Mesozoic displacements of the Indian plate may be associated with the work of an intraoceanic subduction zone and a consuming process under a volcanic island arc now possibly represented by the obducted Kohistan sequence. The spectacular 85-Ma sudden higher wandering rates of India toward Asia would correspond to the beginning of a new process (a new subduction in Tethys (?) with disputed location [4,5,10,18]) so the previously mentioned ages strongly suggest that the Kohistan arc was tectonically welded to the northern edge of India at least 85 Ma ago. If so, it is considered [4,5,10] that the emplacement of the Kohistan arc onto the Indian plate was by an obduction process and it is proposed that D1 in Kohistan, as well as the high-pressure metamorphic [19,20] process under the Kohistan pile, are synchronous and related to this tectonic mechanism.

4. Thermal models during D I and D2: an interpretation (Fig. 4) The metamorphic zones [4] related to the D 1 tectonometamorphic event in Kohistan (Fig. 2) as well as the aspects of the P - T curves (Fig. 3) during this phase suggest the Dl-isotherms are centered on the ortho-derived basic granulites of the Kohistance sequence. It is interresting to point out that the Dl-metamorphism grades not only toward the apparently deepest part of the Kohistan pile but also and mainly toward the granulite belt and the Jijal-Patan complex. At this moment of the discussion and because of the occurrence of these granulites, the question arises whether these features may reflect either the thermal pattern of a unique tectonometamorphic

141 process (the present point of view) or the occurrence of a polymetamorphic-polytectonic system. The pioneer work of Jan [1] considered the granulite belt to be part of a Precambrian basement reactivated during Alpine tectonics whereas the Jijal-Patan complex might be an ophiolitic body representing a fragment of the Tethyan crust. Recently Jan [21] agreed with Bard et al. [4] in suggesting that the Jijal-Patan complex is a layered calc-alkaline pluton (perhaps the top of an ascending partially melted upper mantle-derived diapir) with the same origin as the rocks of the layered granulitic belt, i.e., a large differentiated gabbronoritic intrusion. This interpretation implies that the parental rocks of the granulite unit and the Jijal-Patan complex are comagmatic and that they may form one- or two-layered lopoliths emplaced during a Cretaceous arc-building stage in the Paleotethys. More recently Coward et al. [22] have claimed that the granulitic unit and the Jijal-Patan complex, if effectively comagmatic, represent a single but world-wide unique enormous old layered magmatic body (perhaps much larger than 200 km in north-south direction) affected and redoubled by a large southwards facing D~-recumbent fold. If so, the "sandwiched" occurrence of the granulite belt into amphibolitic rocks would be related to the fact that the granulites have a stratigraphic signification and would outcrop in the hinge of mesozonal pennic-like folds. In our opinion this interpretation is not supported by sufficient and convincing structural data (only some possible overturned cumulative layers very locally situated towards the top of the granulite unit near Chilas). Because the geology of Kohistan is far from completely known, this possibility would be looked upon as an original working hypothesis in actual disagreement: (1) with the present published geochronological data either in Kohistan or the neighbouring Deosai-Ladakh Plateau; (2) with the occurrence of quite different lithology upon and under the hypothetic recumbent fold (major and rare earth element geochemical data) which indicates [51] that the overlying amphibolitic rocks have M O R B / I A B affinities whereas the underlying amphibolites are clearly mostly calc-alkaline and very similar to the granulites they

mainly come from by hydrous retrogressive deep mesozonal shearing and metamorphism (3) with the absence of a large periclinal closure towards the western part of the granulite belt (Tal, Timurgara) (Fig. 1A) that may be related to a large D 1 [22] or D 2 sub-isoclinal mega-recumbent fold. As discussed previously, we consider that the main dynamo-thermal features observed within the granulitic rocks and their neighbouring high-grade amphibolites are sub-synchronous. If so, and because of the magmatic origin of the granulites, the question arises whether theoretical a n d / o r experimental data would be compatible with the conclusion that the thermal structure during D 1 was mainly controlled by still very hot pluton(s) in the Kohistan arc. For this purpose the work of Wells [24] is of a particular interest since this author has predicted that the "distorted" or "sinuous" aspects of the P - T curves ("geotherms") in crustal segment may persist 20 Ma after the emplacement of large magmatic intrusion(s). The intrusion of enormous 1000-1100°C calcalkaline pluton(s) in a volcanic arc must introduce heat source(s) [30,37] that may slowly cool, so that 15 _+ 5 Ma after their emplacement the local "normal geotherms" may or must be still modified. Because the Kohistan granulite belt (i.e., a layered noritic pluton) and the Jijal-Patan complex formed at least 30,000 and 1000 km 3 bodies emplaced around 100 Ma just before the first (D 1 = obduction) 90-85 Ma old tectonometamorphic event, it seems that the D 1 thermal structure affecting the Kohistan sequence could be linked to (a) large calc-alkaline pluton(s) which intruded the Cretaceous volcanic arc some 15-20 Ma before its obduction onto the Indian plate. As discussed by Thompson [23], Well's [24] magmatic theoretical accretionary models provide the most efficient means of simultaneous heating and evolving tectonically thickened crust so as to expose low- to intermediate-pressure facies series at the surface. The Kohistan situation would fit nicely with these models despite some differences the thick basic crust and geodynamic evolution of Kohistan introduce to the type and the time-space of metamorphism. As shown by the Fig. 4B, it is suggested that the thermal structure during D1 was close to

142

the drawn interpretative and speculative aspects of the isoterms in the Dl-deformed Kohistan sequence. The fact that D 1 seems synchronous [16] with the blueschist metamorphism [19] in the MMT suture together with recent petrological and mineralogical studies [2-4], the isotherms throughout the MMT suture are thought to have had the aspects predicted by various models [25-27] for various situations under overthrusting slabs, i.e. a strong narrow downtrending structure of the isotherms between the obducting Kohistan arc (Fig. 4B) and the downgoing Indian plate. Even though a thermal downtrending structure would not be easy to preserve [28] it is suggested that a rapid tectonic climb of the high-pressure metamorphic rocks under or along the obduction main shear zone may have conducted the latter rocks near the surface. From a general viewpoint, it is concluded that the blueschist facies conditions cannot be again convincingly clearly related to early "subduction" stages [29,30] but rather during and perhaps (?) after obduction processes [31-34]. From mineralogical and textural evidences, and partly from the northern areas invaded by numerous syn- to late-D 2 dioritic-granodioritic intrusions (the possible "second" calc-alkaline Kohistan series [10,18]), it seems that the isotherm distribution during D 2 would be a relatively simple one (Fig. 4C). In response to a slow thermal re-equilibration, these isotherms have become flat so they crosscut the earlier metamorphosed calc-alkaline pluton(s) (arc pluton(s)). Assuming the D 2 tectonometamorphic event is Oligocene and synchronous with the first Alpine tectonometamorphic phase in the India plate south of the MMT suture [16], the thermal structure of northern Pakistan 40-50 Ma ago was probably in agreement with possible (?) "folded" isotherms and "inverse" metamorphism [36] in those parts affected by symmetamorphic megashears (i.e. the polyphase MCT shear zone [35]). The downwardtrending aspect of the isotherms in the reactivated M M T suture was probably notably less pronounced (Fig. 4C) than during D 1, so some D 2 mesozonal and epizonal Barrovian-type recrystallisations may have overprinted the earlier blueschist paragenesis.

Acknowledgements The present study was possible after field studies in 1978, 1979 and 1981 with the financial support of the Centre National de la Recherche Scientifique (A.T.P. INAG "Grodynamique II"). EMA were performed with the aid of the latter organism and the Laboratoire de Prtrologie des Z6nes Profondes (University of Montpellier II). I gratefully acknowledge Prof. Dr. Tahirkheli and Prof. Dr. Jan from the Center of Geology of the University of Peshawar for all the facilities they offered to visit the splendid and unique Kohistan geology. I am grateful for very valuable enthusiastic discussions with these colleagues and with the members of the Himalayan team of Montpellier (Dr. Proust, Dr. Maluski, Dr. Matte and Burg). I also thank Dr. Dupuy for pertinent criticism of the petro-chemical grounds of this study. The paper also benefited from perceptive comments by Dr. P.H. Thompson and Dr. B. Harte who also helped to correct the English translation. Mrs. Lanco typed the paper, Mr. Boeuf draw some figures and Mr. Tejedo made thin sections with their usual skill, good nature and efficiency.

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