THE MONTE MAGGIORE PERIDOTITE (CORSICA)

Transcription

1 MONTE MAGGIORE CAPO CORSO CORSICA Giovanni B. Piccardo THE MONTE MAGGIORE PERIDOTITE (CORSICA)

2 FIELD RELATIONSHIPS MORB Gabbro Spinel (ex-garnet) pyroxenites

3 L ESCURSIONE A MONTE MAGGIORE

4 The Monte Maggiore peridotite body and his small satellites are overtrusted on a thin level of prasinites and a level of Centuri gneisses (continental basement of not defined origin). The whole pile is averthrusted on calc-schists and prasinites of the Alpine Corsica. The provenance of the Mt. Maggiore peridotites is not completely understood. They bear mineralogical evidence (jadeite + albite on plagioclase) of having underwent HP evolution during subduction.

5 At Monte Maggiore are present (in structural chronology order from the oldest) : 1) Relics of sub-continental spinel lherzolites with huge pyroxenite bands. Both peridotites and pyroxenites show opx+sp clusters (ex-mantle garnet) which suggest deep provenance (P > 2.5 GPa) (garnet-facies conditions) and exhumation to shallower spinel-facies conditions. 2) Reactive harzburgites which replace sub-continental lherzolites and pyroxenites. They were formed by melt/peridotite interaction of the lithospheric peridotites with MORB-type asthenospheric melts percolating by porous flow and causing pyroxenes dissolution and olivine precipitation at spinel- facies conditions 3) Impregnated plagioclase peridotites which indicate refertilization of the previous reactive spinel harzburgites by MORB-type silica-saturated melts under plagioclase-facies conditions. 4) Gabbro-norites as decametric intrusive pods where they crystallized the silicasaturated melts, strongly depleted in highly incompatible elements. 5) Replacive dunite channels formed by focused percolation of MORB-type silicaundersaturated melts which completely dissolved the peridotite pyroxenes. 6) Gabbroic and basaltic dykes and olivine cumulates pods intruded along fractures (fragile regime) in already serpentinized peridotites

6 STRUCTURAL RELICS OF THE PRISTINE SUB-CONTINENTAL LITHOSPHERIC MANTLE

7 Pyroxenite bands Opx+sp clusters

8 THE REACTIVE PERCOLATION OF SILICA-UNDERSATURATED MORB-TYPE MELTS Pyroxene dissolution olivine precipitation

9 Pyroxenite dissolution and dunite formation

10 Pyroxenite dissolution and dunite formation

11 REACTIVE SPINEL HARZBURGITE

12 PLAGIOCLASE-IMPREGNATED PERIDOTITE Impregnation by silica-saturated melt orthopyroxene formation olivine dissolution

13 Melt impregnation and plagioclase crystallization

14 SOME COMPOSITIONAL DATA OF THE DIFFERENT PERIDOTITE TYPES

15 SiO2 versus MgO bulk rock diagram Dots = spinel peridotites Triangles = plagioclase peridotites 46 1 SiO B C D A MgO Field 2 and 3 = South Lanzo sp- and plg-peridotites Bulk rocks of spinel and plagioclase peridotites plot at lower silica then computed trends (A-B-C-D) for refractory peridotite residua after different types of partial melting. This indicates that they underwent processes different from partial melting (i.e., melt/peridotite reaction during melt percolation) and cannot be considered refractory residua of oceanic melting of the pristine asthenosphere (1)

16 TiO2 versus Cr# in spinel from the different peridotites Field C = South Lanzo reactive spinel peridotites Field B and D = South Lanzo impregnated plg peridotites D TiO B C A Cr#*100 TiO2 is low (< 0.2%) in mantle peridotites (field A), and decreases during partial melting (at increasing Cr# in spinel). At Mt. Maggiore it increases in spinel from reactive spinel peridotites (3) and plagioclase peridotites (4) at increasing Cr#. This indicates that both types of peridotites underwent melt percolation and equilibration of spinel with the percolating melts.

17 REACTIVE PERIDOTITES Spinel-facies fractional melting model 1 Sample/Chondrite 10 1 cpx opx 0,1 0,01 0,001 La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Y Er Tm Yb Lu Sample/Chondrite REE pattern of liquid in equilibrium with cpx 0.01 La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Plg IMPREGNATED PERIDOTITES Spinel-facies fractional melting model Sample/Chondrite 100 cpx 10 opx 1 plg La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Y Er Tm Yb Lu Sample/Chondrite REE pattern of liquid in equilibrium with cpx 0.01 La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

18 THE REPLACIVE DUNITES

19 Replacive dunites following plagioclase impregnation

20 Interstitial melt crystallization in replacive dunites

21 INCEPTION OF MELT CRYSTALLIZATION Formation of gabbro-norite dykelets and gabbro-norite intrusive pods

22 Gabbro-norite dykelets Gabbro-norites stratified pods

23 Replacive dunites following plagioclase impregnation

24 SOME COMPOSITIONAL FEATURES OF GABBRO-NORITES

25 The Monte Maggiore gabbro-norite rocks are characterized by forsteritic olivine (Fo 90), very anothitic plagioclase (An ) and Mg-rich pyroxenes (Cpx Mg# ; Opx Mg# 91.8). Clinopyroxenes show high Cr 2 O 3 ( wt%) and low TiO 2 ( wt%) and Na 2 O ( wt%). They have very low LREE (La N = ), Sr ( ppm) and Zr ( ppm) contents. Their C1-normalized REE patterns (Fig. 5) are rather flat in the HREE-MREE region (at about 8-15xC1) and are strongly LREE fractionated (La N /Sm N = ). Orthopyroxenes are rich of Cr (Cr 2 O wt%). Plagioclase has very low LREE (e.g. Ce N = ) and Sr ( ppm): C1-normalized REE patterns show a negative LREE fractionation (La N /Sm N = ). The Monte Maggiore dykelets are characterized by rare forsteritic olivine (Fo 90), very anorthite-rich plagioclase (An 91.5) and Mg-rich pyroxenes (Cpx Mg# ; Opx Mg# 89.7). Clinopyroxenes have high Cr 2 O 3 ( wt%) and are Ti- and Na-poor (TiO wt% and Na 2 O wt%). They are also characterized by very low La N ( ), Sr ( ppm) and Zr ( ppm) contents. Their C1-normalized REE patterns are rather flat in the HREE-MREE region (at about 8-11xC1) and are strongly LREE fractionated (La N /Sm N = ) (Fig. 6). Plagioclase has very low contents of Ce N ( ) and Sr ( ppm): their C1-normalized REE patterns show a negative LREE fractionation (La N /Sm N = ).

26 A = oceanic (MOR) gabbro-norites 3 An% (plg) B 6 Na 2 O (cpx) B 6 A Mg# (cpx) Mg# (cpx) Sr (plg) ppm B A = gabbro-norites B = gabbroic dykes An % (plg) 1 A Sr (cpx) A B Zr (cpx)

27 A 100 GABBO CR9/1 10 Cpx Opx Cpx/C1 1 Plg GABBRO-NORITES B Cpx/C La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Y Dykelets CR9/1 =Cpx in equilibrium with the less evoved melt Plg Er Tm Yb Lu Cpx REE patterns of minerals from gabbronorite pods (A) and dykelets (B): pyroxenes are strongly fractionated in the LREE and plagioclase has fractionated LREE from Sm to Ce), suggesting strong depletion in incompatible trace elements of the parental melts La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Y Er Tm Yb Lu

28 COMPOSITION OF THE PARENTAL LIQUID OF GABBRO-NORITES When calculated the liquid in equilibrium with the core composition of the cpx (first crystallizates from the more primitive melt) of the most primitive gabbronorite sample CR 91, REE pattern of equilibrium melt well fits with the modelled composition of liquids in equilibrium with cpx obtained by 5-7% degrees of fractional melting of a spinel-facies DM mantle source. 1% 100 1% Sample/Chondrite % 10% 0.01 La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

29 THE HIDDEN MAGMATISM Our evidence indicate tha an early asthenospheric magmatic cycle involved the extending mantle lithosphere as a conseuence of the porous flow percolation of single melt fractions formed when the almost adiabatic upwelling asthennospher underwent partial melting on decompression (most probably after 50% of reduction of the litosphere during extention and thinning). The early asthenospheric melts were MORB-type single melt increment of fractional melting (5-7%), strongly depleted in incompatible elements (and particularly LREE) and survived isolated when percolating. Pyroxene dissolution and olivine precipitation during early reactive melt ercolation led to silica-saturation of the melts, which impregnated the plagiocase peridotites. Gabbro-norite formation was the last stage of this magmatism. these melts never reached as basaltic lavas the sea-floor.

30 cumulates from strongly depleted MOR liquids were described at DSDP Site 334 at MAR (Hodges and Papike, 1976; Ross and Elthon, 199 Ross Peculiar and Elthon cumulates (1993) evidenced that from fractional strongly melting of the depleted upwelling sub-oceanic MOR mantle liquids produces were magmas described with a much wider at range DSDP of compo Site 334 at MAR (Hodges and Papike, 1976; Ross and Elthon, 1993). These unique suite of oceanic cumulates are plagioclase lherzolites, olivine gabbronorites, gabbro-norites and noritic anorthosites that contain olivine (Fo 90-85), high-ca and low-ca pyroxenes (Mg# 91-70) and plagioclase (An 90-75). They have anomalously high anorthite content in plagioclase and very high Mg# and Cr abundances in pyroxenes, coupled with low Na and Ti in high-ca pyroxenes and very low incompatible element abundances in pyroxenes. Orthopyroxene is an abundant cumulus phase in the gabbro-norites. Abundance and early crystallization of magnesian orthopyroxene suggests that parental magmas of Site 334 cumulates were high silica (52-55 wt%) liquids. According to Ross and Helton (1993) the mineral compositions of this unique suite of oceanic cumulates indicate that these rocks crystallized from basaltic liquids that were strongly depleted in Na, Ti, Zr, Y, Sr and REE relative to any erupted MORB. This suite of cumulates provides supporting evidences for the existence of strongly depleted liquids in the oceanic lithosphere and for the fact that such magmas did not mix with more enriched MORB liquids and remained sufficiently isolated to form distinctive cumulates (Ross and Elthon, 1993).

31 Ross and Elthon (1993) evidenced that fractional melting of the upwelling suboceanic mantle produces magmas with a much wider range of compositions than erupted MORBs and that strongly depleted primary magmas are routinely produced by melting beneath ridges (e.g. Johnson et al., 1990). Such magmas remained sufficiently isolated to form distinctive intrusive rocks. These authors remarked that the absence of strongly depleted melts as erupted lavas prompts the question of how long such magmas survive before their distinctive compositions are erased by mixing with more enriched magmas to give aggregated MORB.

32 THE GABBROIC-BASALTIC INTRUSIONS GABBROIC DYKES OLIVINE CUMULATE PODS BASALTIC DYKES WERE INTRUDED ALONG FRACTURES The host peridotites were frequently already serpentinized

33 Gabbroic dykes Basaltic dykes

34 Olivine cumulate pods

35 SOME COMPOSITIONAL FEATURES OF GABBROIC ROCKS

36 The Monte Maggiore gabbroic dykes The Monte Maggiore gabbroic dykes are characterized by olivine (Fo = ), plagioclase (An = ), and clinopyroxene (Mg# = ). Positively correlations between Fo in olivine, Mg# in clinopyroxene and An in plagioclase are observed, thus indicating that the different samples crystallized from progressively more evolved magmas. Rare Ti-magnetite crystals are present in samples showing the lower Fo, Mg#cpx and An contents, indicating transition to Fe-Ti-oxide gabbroic compositions. The bulk rock C1-normalized REE patterns of the gabbroic dykes are almost flat in the MREE-HREE region, showing slight LREE fractionation and positive Eu N anomaly that suggests early cumulus of plagioclase, following the crystallization order. The bulk rock C1-normalized REE patterns are subparallel for this suite of rocks. Their absolute REE concentrations increase concordantly with decreasing Fo in olivine, An in plagioclase and Mg# in clinopyroxene, in agreement with progressive effects of melt evolution by low pressure crystal fractionation of the parental melts.

37 A = oceanic (MOR) gabbro-norites 3 An% (plg) B 6 Na 2 O (cpx) B 6 A Mg# (cpx) Mg# (cpx) Sr (plg) ppm B A = gabbro-norites B = gabbroic dykes An % (plg) 1 A Sr (cpx) A B Zr (cpx)

38 Clinopyroxenes have compositions varying from diopside (in the more primitive rocks) to augite (in the more evolved rocks) and are relatively enriched in incompatible trace elements, i.e., Sr ppm and Zr ppm. They show overall C1-normalized REE patterns characterized by a significant LREE negative fractionation and slight negative Eu N anomaly. The less evolved M28 dyke, showing the highest Mg content in olivine (Fo 81.4), has C1-normalized REE pattern almost flat in the MREE-HREE region, showing the lower REE absolute concentration (at about 15xC1 in the MREE- HREE region), slight LREE fractionation (La N /Sm N = 0.2) and no Eu N anomaly. The clinopyroxene C1-normalized REE patterns of the gabbroic dykes reach absolute concentrations of about 50xC1 in the MREE-HREE region in the most evolved samples. Plagioclases are relatively rich in Sr ( ppm) and have normalized REE patterns showing significant positive Eu N anomaly and positive LREE fractionation, similarly to plagioclase in equilibrium with MORB. Concordantly with clinopyroxenes, the absolute REE concentrations increase with increasing degrees of evolution of the host rock. Mineral chemistry features of the gabbroic dykes indicate that they are consistent with cumulate rocks produced by fractional crystallization from variably evolved magmas most probably formed by similar primary liquids.

39 GABBROIC Gabbroic dykes, INTRUSIONS olivine cumulates and basaltic dykes were intrused at very shallow levels close to the sea-floor and can be related to the MORB magmatism, gabbros and pillowed basalts), of the basin. Compositional data suggest that these melts were typical aggregated MORB. They underwent low pression fractional crystallization into ephemeral magma chambers and formed variably differentiated Al- Mg and Fe-Ti liquids, that are present as dykes of variable Mggabbros and Fe-Ti-gabbros. Whole Roccia rock totale Minerals cpx Sample/Chondrite 10 1 Sample/Chondrite 10 1 plg La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Y Er Tm Yb Lu 0.01 La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Y Er Tm Yb Lu

40 GEOCHRONOLOGICAL DATA 0, Nd/ 144 Nd 0,5136 0,5135 0,5134 0,5133 0,5132 MM-sp-perid MM-pl-perid MM-pl-perid MM-veinlet MM-gabbro MM-gabbro 0,5131 0,5130 0,5129 0,0 0,2 0,4 0,6 0,8 147 Sm/ 143 Nd Plagioclase whole rock clinopyroxene data for a gabbro-noritic veinlet and a gabbro dyke define internal isochrons yielding Jurassic ages (155±6 Ma and 162±10 Ma, respectively), and initial εnd = 8.9 and 9.7, consistent with a MORB affinity. Peridotite model ages are concordant with this trend. All the ages are essentially coincident within the analytical uncertainty, and indicate that melt impregnation and subsequent gabbroic intrusion occurred at Middle-Late Jurassic times during the rifting stages in the Ligure-Piemontese system.

41 DISCUSSION AND CONCLUSION

42 1000 Sample/Chondrite [3] N-MORB after Hofmann (1988) 0.01 La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Our data on the Monte Maggiore peridotites and related rocks reveal the subcontinental lithosphere protoliths of the pristine spinel (ex-garnet) peridotites and evidence the presence of two magmatic cycles, during the rifting and oceanic stage of the Late Jurassic Ligurian-Piemontese basin.

43 THE HIDDEN MAGMATISM The first magmatic cycle was represented by melt percolating by porous flow of MORB-type isolated single melt fractions upwelling from the asthenosphere after variable degrees (5-7%) after fractional melting at spinel-facies conditions. These melt fractions were stronlgy depleted in incompatible trace elements and percolated reactively though the spinel-facies mantle lithosphere, dissolving pyroxenes and precipitating olivine. These melts became saturated in silica as a consequence of the melt/peridotite reaction, percolated and crystallized interstitially at plagioclase-facies conditions, forming refertilized plagioclase peridotites. Loclally, they formed pods and dykelets of gabbro-norites, characterized by extreme incompatible trace element depletion. Interstitial crystallization in peridotite and in intrusive pods caused stagnation and entrapment of these melts in the shallow mantle lithosphere: these melts never reached the sea-floor of the basin. No basalts with these compositions have been described as pillow-lavas or dykes on the sea-floor. The formation of gabbro-norite cumulates marked the change of the melt migration mechanism from diffuse porous flow percolation to intrusion and crystallization when cooling by conducive heat loss became dominant on heating by melt percolation.

44 THE OCEANIC MAGMATISM Progressive upwelling and cooling of the host peridotite during rifting caused transition to more fragile conditions and, sporadically, to hydration and serpentinization. The peridotite body was then intruded along fractures by variably evolved, Mg- Al- to Fe-Ti-rich gabbroic dykes, that show distinctive compositional characteristics with respect to the early gabbro-norites. Computed melt compositions in equilibrium with clinopyroxenes from less evolved gabbros are closely similar to aggregated MORBs indicating thus the first upwelling of aggregated MORB melts through the mantle column. The event of aggregated MORB gabbro intrusion indicates that: i) the melt dynamics in the asthenosphere were changed and the isolated melt fractions were more efficiently mixed to form aggregated MORB, ii) ii) MORBs most probably migrated through the mantle lithosphere without significant interaction with the host peridotite, and iii) iii) upwelling melts should have stagnated into shallow ephemeral magma chambers were underwent variable degrees of evolution to form the different parental magmas of the different gabbroic dykes.

45