request. Permission to seek permission to re-use all or part of this article ... and acid intrusive rocks from a number of continent-continent collision zones of Phanerozoic age. ... Since plate collision is a ...... collision, (b) syn-collision, (c) post- to.
Geological Society, London, Special Publications Geochemical characteristics of collision-zone magmatism Nigel B. W. Harris, Julian A. Pearce and Andrew G. Tindle Geological Society, London, Special Publications 1986; v. 19; p. 67-81 doi:10.1144/GSL.SP.1986.019.01.04
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© 1986 Geological Society of London
Geochemical characteristics of collision-zone magmatism N. B. W. Harris, J. A. Pearce & A. G. Tindle SUMMARY: This paper reports the results of a systematic geochemical study of intermediate and acid intrusive rocks from a number of continent-continent collision zones of Phanerozoic age. Four groups of intrusions can be recognized, each associated with a particular stage in the tectonic evolution of a collision zone. (i) Pre-collision calc-alkaline (volcanic-arc) intrusions which are mostly derived from mantle modified by a subduction component and which are characterized by selective enrichments in LIL elements. (ii) Syn-collision peraluminous intrusions (leucogranites) which may be derived from the hydrated bases of continental thrust sheets and which are characterized by high Rb/Zr and Ta/Nb and low K/Rb ratios. (iii) Late or post-collision calc-alkaline intrusions which may be derived from a mantle source but undergo extensive crustal contamination and can only be distinguished from volcanic-arc intrusions by their higher ratios of Ta/Hf and Ta/Zr. (iv) Post-collision alkaline intrusions which may be derived from mantle lithosphere beneath the collision zones and which carry high concentrations of both LIL and HFS elements. The geochemical evolution of crustal melts within groups (ii) and (iii) can be viewed in terms of the dehydration reactions, volatile transfer and transient geothermal gradients that result from thrust tectonics in the zone of collision.
A b r o a d e r study of the geochemical characteristics of intermediate-acid intrusive rocks from a range of tectonic settings indicates that, in m a n y cases, trace e l e m e n t s may be used as discriminants for the tectonic setting in which the m a g m a was i n t r u d e d (Pearce et al. 1984). O n a Rb-Hf-Ta triangular plot (Fig. 1), for example, intrusives from volcanic-arc, withinplate and ocean-floor settings occupy t h r e e fields with little overlap. H o w e v e r , intrusives from k n o w n collision settings, if taken as a single group, overlap with the fields of volcanic-arc and, to a lesser extent, withinplate intrusions. Since plate collision is a dynamic event which evolves from an initial stage of oceanic lithosphere subduction t h r o u g h a period of continental o r o g e n y and crustal thickening to a period of stabilized continental lithosphere, a range of source regions for collision m a g m a s is implied and overlap with o t h e r tectonic settings is predictable. In this paper we examine m o r e closely the relationship b e t w e e n the geochemistry and tectonic setting of collision magmatism. W e first describe our sources of data on collision intrusions and h o w the data can be divided into four groups on the basis of their t e m p o r a l and spatial settings within collision zones. W e t h e n c o m p a r e the compositions of these groups in order to identify how the source region, or range of source regions, for m a g m a genesis might vary during the collision event.
Rb/lO
Hf
Ta x3 FIG. 1. Rb-Hf-Ta triangular plot for acidintermediate intrusive magmatism from volcanic-arc (triangles), ocean-floor (diamonds), within-plate (squares) and collision (circles) tectonic settings. For sources of data, see Pearce et al. (1984).
Sources of geochemical data A data set of 115 geochemical analyses of intermediate-acid intrusive rocks from regions of collision tectonics has been collated from the literature and from u n p u b l i s h e d data analysed at the O p e n University ( s e v e n t e e n
From COWARD,M. P. & RIES, A. C. (eds), 1986, Collision Tectonics, Geological Society Special Publication No. 19, pp. 67-81.
67
68
N.B.W.
Harris et al.
analyses). Most of these come from three areas of continent-continent collision (the Himalayas, the Alps and the Hercynides of SW Europe) which, being also best understood in terms of their broad tectonic evolution, form the basis of this work. However, data from other collision zones, including the Triassic zones of SE Asia, the Palaeozoic zones of the Caledonides, the Pan-African zones of N Africa and Arabia and the Proterozoic zone of the Baltic Shield, have been used to test the general applicability of any models based on these three areas. The details of the data sources are as follows. The Himalayas
Himalayan intrusives have been broadly divided into four suites (Debon et al. 1981): 1 The Trans-Himalayan plutons (also known as the Gandise granitoids) are intruded in an E - W belt which lies N of the Tsangpo Suture on the southern edge of the Eurasian Plate. These are Upper Cretaceous to Eocene in age (100-50 Ma) and pre-date collision between the Indian and Eurasian Plates. Trace-element data have been published from the Ladakh region of the belt (Honnegar et al. 1982). 2 The High Himalayan leucogranites are emplaced between the Chiatsun Thrust to the N and the Main Central Thrust to the S on the Indian Plate (see Shackleton 1981 for map and structural setting). These syntectonic intrusions are Eocene to Miocene in age (50-10 Ma) and intrude the Palaeozoic and Mesozoic metasediments of the High Himalayas. They are represented in the trace-element data bank by published data from the Manaslu Granite (Cocherie 1978; Vidal et al. 1982), from the Bhutan granites (Dietrich & Gansser 1981) and by unpublished data from the Gabug pluton in Southern Tibet (see Jun-wen et al. 1981 for description). 3 The Llagoi Kangri granite belt lies between the Chiatsun Thrust and the Tsangpo Suture and therefore between the Trans-Himalayan Belt and the High Himalayan leucogranites. Analyses from this belt have not been included in this study since ambiguous age determinations from Llagoi Kangri granites suggest they may be Lower Palaeozoic in age and therefore have source regions unrelated to the Himalayan collision event. Le Fort (this volume) suggests that these granites may represent Palaeozoic gneiss domes which were reactivated during the Himalayan collision event. 4 The Lesser Himalayan granites form the
most southern granitoid belt S of the Main Central Thrust. Their Cambrian age (Le Fort et al. 1980), and hence unknown intrusive setting, precludes them from this study. The Alps
The characteristic granitic magmatism of the Alps is an Oligocene suite of calc-alkaline intrusives emplaced along the Alpine Chain (Exner 1976). The largest granitic body is the Adamello Massif which intrudes Alpine crystalline basement in the Southern Alps between the Insubric Line to the N and the Gindicaric Line to the SE (Dupuy et al. 1982). In the Eastern Alps the Vedrette di Ries (Riesserferner) and the Cima di Vila (Zinshock) plutonic complexes intrude the crystalline basement S of the Tauern Window and N of the Deferegger-Anterselva-Valles Line (Bellieni et al. 1981; Bellieni 1982). North of the Insubric Line the Bergell suite intrudes Penninic nappes in the Central Alps. Also in the Bergell area the Novate two-mica granite post-dates the Bergell suite (Gulson & Krogh 1973). The Bergell intrusives are represented in the data bank by unpublished data. Both the calc-alkaline and the micaceous Alpine granites are Oligocene in age (30-:25 Ma) thus post-dating the Cretaceous collision event by - 4 0 Ma (Hawkesworth et al. 1975). The Hercynides of SW Europe
Hercynian granites in SW Europe lie in a belt - 3 0 0 0 k i n long and 1000kin wide, stretching from Portugal to the Carpathians (Laurent 1972). These intrusions may be divided into two groups: an earlier phase of syn-tectonic granites, which were emplaced at around 300-325 Ma; and a later phase of late to post-tectonic granitoids, which were emplaced at 260-280 Ma (Didier & Lameyre 1969). Data for the first group have been taken from Galicia, NW Spain (Cocherie 1978), the Barousse Massif, French Pyrenees (Harris 1973 and unpublished data) and Cornwall (Alderton et al. 1980). Geochemical data for the second group are taken from intrusions in the French Pyrenees: the Querigut (Fourcade & Allegre 1981) and Barousse (unpublished data) massifs, Galicia (Cocherie 1978) and Northern Portugal (Albuquerque 1971, 1978). A third magmatic phase of post-tectonic alkaline granites and syenites has been identified (Fourcade & Allegre 1981; Brotzu et al. 1978) but no analyses of these intrusions are yet available.
Collision-zone magmatism
Other collision zones Other zones of continent-continent collision also appear to demonstrate a complex sequence of granite emplacement. The data bank includes analyses of such collision-related intrusions from the Triassic An Ding Suture in SW China (unpublished data), the Caledonian orogenic belt of Newfoundland and Scotland (Tindle & Pearce 1981 and unpublished data), the Pan-African orogenic belt (Harris 1982; Radain & Fyfe 1982) and the Svecofennian orogenic belt (Pharoah & Pearce 1984). These intrusions show many similarities in setting with those from the three regions desscribed above but, because of the relative lack of tectonic control, they form only a subsidiary part of this study.
Classification of collision m a g m a t i s m The petrology, field relations and geochronology of intrusions found in the Himalayas, Alpine and Hercynian collision zones points to a possible 4-fold classification of collision magmatism. Group-I intrusions pre-date collision and are represented by the Trans-Himalayan granitic suite. They typically form a high-level calcalkaline suite ranging from gabbros to biotite granites, in which diorites, tonalites and granodiorites are the dominant rock types. Group-I intrusions are similar in field relations, mineralogy and geochemistry to intrusions found in active continental margins and are thus assumed to be of volcanic-arc origin. They are represented in the Himalayas by the Trans-Himalayan granitic suite, are rare or absent in the Alpine and Hercynian Belts but are also present in other collision zones in variable abundances. Group-ll intrusions are syn-orogenic granites which are commonly known as leucogranites. They produce conformable or semiconformable intrusions which often contain pelitic enclaves and are broadly similar to the S-type granite group from the Australian classification (Chappell & White 1974). Mineralogically they are characterized by the presence of muscovite, with or without biotite, and tourmaline is also common. Silica contents of Group-II intrusions usually exceed 70 wt%. They are emplaced within metamorphic terranes which often include migmatites. Granites of this type are represented by the High Himalayan leucogranites and the syntectonic granites of the Hercynides, but are
69
rare in the Alps. They are exposed in some older collision belts including the Newfoundland Caledonides (Strong & Dickson 1978) and the Triassic zones of SE Asia (Hutchinson 1977). Migmatite leucosomes have not been included in this group since, although they have similar mineralogy and structural relationships, they differ significantly in scale and in chemical composition. Group-III intrusions form calc-alkaline suites ranging from gabbros through to granites but are dominantly biotite-hornblende tonalites and granodiorites. They are emplaced as high-level intrusions with sharp cross-cuiting contacts and they frequently contain enclaves of intermediate, basic and even ultrabasic plutonic rocks. Despite their petrographic similarities they differ from Group I in having been emplaced after the collision event. Intrusions of this type are represented by the post-tectonic granitoids in the Alps and by the post-tectonic Hercynian suites of the Iberian Peninsula and the satellite massifs of the Northern Pyrenees. They are also common in most of the other orogenic belts studied, notably the Scottish Caledonides and the Svecofennian orogenic belts. Group-IV intrusions form minor hypabyssal or high-level plutonic suites. In the Alps, alkali basaltic and shoshonitic dykes are emplaced in a post-collision environment (Deutsch 1980). In the Hercynides, post-tectonic alkaline granites and syenites were emplaced in the Pyrenees (Fourcade & Allegre 1981; Brotzu et al. 1978) and a Cretaceous alkaline province, including lamprophyric and nepheline syenitic minor intrusions, was developed in Western Iberia and the Pyrenees (Rock 1982).
G e o c h e m i c a l characteristics Group 1 These intrusions are of volcanic-arc type since collision had not occurred at the time of their formation. Thus trace-element data from the Ladakh region of the Trans-Himalayan Batholith (Honnegar et al. 1982) and from pre-collision intrusions in Arabia (Harris 1982), demonstrate selective enrichment in LIL elements which is thought to be typical of magmas derived from mantle modified by a subduction component. The intrusions of Ladakh and Southern Tibet have low initial 87Sr/86Sr ratios (ESr = +2 to - 2 0 ) which supports this interpretation (Table 1, Fig. 2).
70
H a r r i s et al.
N.B.W.
TABLE 1. I s o t o p i c data f r o m collision-related granites ESr
Group I
Himalayas
(Quxu, Tibet)~ (Ladakh) 2"3
Group II
Himalayas
(Gabug, Tibet) 1 (Manaslu) 3 (Makalu) 3 (Nuptse) 4 (Lhotse) 4
Group III
ENd
- 15/- 20 + 2
-
+612 +513
1
-12 -9
+649 +405
Pyrenees
(Aix Les Thermes) 3 (Barousse) 5 (Aston-Hospitalet) 6 (Maladeta) 7
+ 95 + 151 +216 + 105/+ 152
Massif Central
(Montagne Noire) 8
NW Spain
(Galicia) 9
+79/+ 109
Pyrenees
(Querigut) 3 (Costabonne) 3 (Barousse) 5 (Maladeta) 7
+28/+ 155 + 94 + 51 + 104
Massif Central
(Montagne Noire) 8
NW Spain
(Galicia) 9
+66/+ 194
Alps
(Adamello) m (Vedrette di Ries) 11
+5/+ 106 +46/+ 88
-8
+ 81 0/- 8 -3
+ 47
1 Jun-wen et al. 1981; 2 Honnegar et al. 1982; 3 Allegre & Ben Othman 1980; 4 Ferrara et al. 1983; 5 Harris 1973; 6 Jager & Zwart 1968; 7 Vitrac et al. 1980; 8 Hamet & Allegre 1976; 9 Cocherie 1978; 10 Cortecci et al. 1979; 11 Borsi et al. 1979.
--100 l
0 I
.1.100 i
.1.200 I
.1.300 I
.1.400 l
[]
Modern Volcanic Arcs
[]
Group I (Himalayas}
.1.500 I
-t-600 I
-I-700 I
Sr
Group II
E::::::::::::::::::::::::::::::,:::::::::::?:::::::::(Himalayas) ::::::::::::::• Group II (Hercynides) |~iiiii~iiii::l Group Iii (Hercynides} Group III (Alps} E~i~i~i~:::.~i~l
Fro. 2. ~s, from collision-zone magmatism. For source of data see Table 1. Volcanic-arc data from Hawkesworth (1979).
Group
II
G r o u p - I I granites are generally e n r i c h e d in R b and possibly Ta but d e p l e t e d in h e a v y rare earth e l e m e n t s ( H R E E ) , Y, Zr and Hf, in c o m p a r i s o n with volcanic-arc m a g m a s (Fig. 3, Table 2). G r o u p - I I m a g m a s are t h e r e f o r e c h a r a c t e r i z e d by high ratios of Rb/M w h e r e M m a y be H R E E , Y, Z r or Hf. O n e of the most effective discriminants is the R b / Z r ratio which separates Group-II from G r o u p - I l I and volcanic-arc m a g m a s (Fig. 4). This plot de-
m o n s t r a t e s the restricted, high SiO 2 range of G r o u p - I I granites. T h e r e is also some e v i d e n c e to suggest that G r o u p - l I granites m a y exhibit unusually high ratios of Ta to Nb. On a diagram of Ta against Nb (Fig. 5), granites of volcanic-arc and withinplate origin plot within a n a r r o w b a n d indicating little variation in Ta/Nb ratios (about 1/12 on average) despite the large variation in the absolute a b u n d a n c e s of the two e l e m e n t s . In contrast most G r o u p - I I granites lie above this band. H o w e v e r , it must be e m p h a s i z e d
Collision-zone magmatism
71
TABLE 2. Representative analyses from Group-H granites Alps
Hercynian of SW Europe
Himalayas Manaslu 1 (Nepal)
Manaslu I aplite (Nepal)
Bhutan 2
Gabug 3 (Tibet)
Barousse 3 (Pyrenees)
Galicia 4 (NW Spain)
Cornwall 5
Novate (Italian Alps)
73.05 0.15 14.59 1.19 * 0.03 0.11 0.59 3.63 4.92 --
75.04 0.05 14.19 0.67 * 0.04 0.10 0.06 4.38 4.85 --
73.16 0.11 14.52 0.19 0.90 0.04 0.14 0.74 3.04 4.98 0.16
73.17 0.06 15.42 0.62 * 0.01 0.16 0.62 3.96 4.56 0.15
73.89 0.07 15.80 0.26 * 0.00 0.20 1.14 4.37 3.88 0.16
71.90 0.25 14.20 2.25 * 0.02 0.46 1.00 3.33 5.35 0.16
71.73 0.25 14.55 1.38 0.72 0.03 0.46 0.69 2.52 5.50 0.23
76.20 0.24 14.21 0.28
Wt%
SiO2 TiO2 A1203
Fe203 FeO MnO MgO CaO
Na20 K20 P205 Ppm Rb Ba Sr
258 415 131
Zr Y Nb Th Ta Hf
69 --5.6 3.2 2.2
La Ce Nd Sm Eu Gd Tb Yb Lu
13.9 28.0 14.6 3.3 0.78 3.4 0.70 0.62 0.083
569 16 7.52
373 243 62
